Remove database specific sql statements from SQLCounter

Every database executes different type of sql statement to get metadata then `ActiveRecord::TestCase` ignores these database specific sql statements to make `assert_queries` or `assert_no_queries` work consistently.

Connection adapter already labels these statement by setting "SCHEMA" argument, this pull request makes use of "SCHEMA" argument to ignore metadata queries.

Here are the details of these changes:

* PostgresqlConnectionTest

Each of PostgresqlConnectionTest modified just executes corresponding methods

https://github.com/rails/rails/blob/fef174f5c524edacbcad846d68400e7fe114a15a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb#L182-L195

```ruby
        # Returns the current database encoding format.
        def encoding
          query_value("SELECT pg_encoding_to_char(encoding) FROM pg_database WHERE datname = current_database()", "SCHEMA")
        end

        # Returns the current database collation.
        def collation
          query_value("SELECT datcollate FROM pg_database WHERE datname = current_database()", "SCHEMA")
        end

        # Returns the current database ctype.
        def ctype
          query_value("SELECT datctype FROM pg_database WHERE datname = current_database()", "SCHEMA")
        end
```

* BulkAlterTableMigrationsTest

mysql2 adapter executes `SHOW KEYS FROM ...` to see if there is an index already created as below. I think the main concerns of these tests are how each database adapter creates or drops indexes then ignoring  `SHOW KEYS FROM` statement makes sense.

https://github.com/rails/rails/blob/fef174f5c524edacbcad846d68400e7fe114a15a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb#L11

```ruby
          execute_and_free("SHOW KEYS FROM #{quote_table_name(table_name)}", "SCHEMA") do |result|
```

* Temporary change not included in this commit to show which statements executed

```diff
$ git diff
diff --git a/activerecord/test/cases/migration_test.rb b/activerecord/test/cases/migration_test.rb
index 8e8ed494d9..df05f9bd16 100644
--- a/activerecord/test/cases/migration_test.rb
+++ b/activerecord/test/cases/migration_test.rb
@@ -854,7 +854,7 @@ def test_adding_indexes

       classname = ActiveRecord::Base.connection.class.name[/[^:]*$/]
       expected_query_count = {
-        "Mysql2Adapter"     => 3, # Adding an index fires a query every time to check if an index already exists or not
+        "Mysql2Adapter"     => 1, # Adding an index fires a query every time to check if an index already exists or not
         "PostgreSQLAdapter" => 2,
       }.fetch(classname) {
         raise "need an expected query count for #{classname}"
@@ -886,7 +886,7 @@ def test_removing_index

       classname = ActiveRecord::Base.connection.class.name[/[^:]*$/]
       expected_query_count = {
-        "Mysql2Adapter"     => 3, # Adding an index fires a query every time to check if an index already exists or not
+        "Mysql2Adapter"     => 1, # Adding an index fires a query every time to check if an index already exists or not
         "PostgreSQLAdapter" => 2,
       }.fetch(classname) {
         raise "need an expected query count for #{classname}"
$
```

* Executed these modified tests

```ruby
$ ARCONN=mysql2 bin/test test/cases/migration_test.rb -n /index/
Using mysql2
Run options: -n /index/ --seed 8462

F

Failure:
BulkAlterTableMigrationsTest#test_adding_indexes [/home/yahonda/git/rails/activerecord/test/cases/migration_test.rb:863]:
3 instead of 1 queries were executed.
Queries:
SHOW KEYS FROM `delete_me`
SHOW KEYS FROM `delete_me`
ALTER TABLE `delete_me` ADD UNIQUE INDEX `awesome_username_index`  (`username`), ADD  INDEX `index_delete_me_on_name_and_age`  (`name`, `age`).
Expected: 1
  Actual: 3

bin/test test/cases/migration_test.rb:848

F

Failure:
BulkAlterTableMigrationsTest#test_removing_index [/home/yahonda/git/rails/activerecord/test/cases/migration_test.rb:895]:
3 instead of 1 queries were executed.
Queries:
SHOW KEYS FROM `delete_me`
SHOW KEYS FROM `delete_me`
ALTER TABLE `delete_me` DROP INDEX `index_delete_me_on_name`, ADD UNIQUE INDEX `new_name_index`  (`name`).
Expected: 1
  Actual: 3

bin/test test/cases/migration_test.rb:879

..

Finished in 0.379245s, 10.5473 runs/s, 7.9105 assertions/s.
4 runs, 3 assertions, 2 failures, 0 errors, 0 skips
$
```

* ActiveRecord::ConnectionAdapters::Savepoints

Left `self.ignored_sql` to ignore savepoint related statements because these SQL statements are not related "SCHEMA"

```
self.ignored_sql = [/^SAVEPOINT/, /^ROLLBACK TO SAVEPOINT/, /^RELEASE SAVEPOINT/]
```

https://github.com/rails/rails/blob/fef174f5c524edacbcad846d68400e7fe114a15a/activerecord/lib/active_record/connection_adapters/abstract/savepoints.rb#L10-L20

```ruby
      def create_savepoint(name = current_savepoint_name)
        execute("SAVEPOINT #{name}")
      end

      def exec_rollback_to_savepoint(name = current_savepoint_name)
        execute("ROLLBACK TO SAVEPOINT #{name}")
      end

      def release_savepoint(name = current_savepoint_name)
        execute("RELEASE SAVEPOINT #{name}")
      end
```

1 files changed, 3 insertions, 3 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/connection_test.rb b/activerecord/test/cases/adapters/postgresql/connection_test.rb
index 210758f462..9494863a75 100644
--- a/activerecord/test/cases/adapters/postgresql/connection_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/connection_test.rb
@@ -26,19 +26,19 @@ module ActiveRecord
     end
 
     def test_encoding
-      assert_queries(1) do
+      assert_queries(1, ignore_none: true) do
         assert_not_nil @connection.encoding
       end
     end
 
     def test_collation
-      assert_queries(1) do
+      assert_queries(1, ignore_none: true) do
         assert_not_nil @connection.collation
       end
     end
 
     def test_ctype
-      assert_queries(1) do
+      assert_queries(1, ignore_none: true) do
         assert_not_nil @connection.ctype
       end
     end