Improve the performance of writing attributes

Using a similar approach to 08576b94ad4f19dfc368619d7751e211d23dcad8,
this change adds a new internal `_write_attribute` method which bypasses
the code that checks for attribute aliases and custom primary keys.

We can use this method instead of `write_attribute` when we know that we
have the name of the actual column to be updated and not an alias.

This makes writing an attribute with `attribute=` about 18% faster.

Benchmark:

```
begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", github: "rails/rails"
  gem "arel", github: "rails/arel"
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end
end

class Post < ActiveRecord::Base
end

post = Post.new(id: 1)

Benchmark.ips do |x|
  x.report("attribute=") { post.id = post.id + 1 }
end
```

Before:

    Warming up --------------------------------------
              attribute=    25.889k i/100ms
    Calculating -------------------------------------
              attribute=    290.946k (± 3.1%) i/s -      1.476M in   5.077036s

After:

    Warming up --------------------------------------
              attribute=    30.056k i/100ms
    Calculating -------------------------------------
              attribute=    345.088k (± 4.8%) i/s -      1.743M in   5.064264s

4 files changed, 14 insertions, 8 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index b9b2acff37..081aad434d 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -21,7 +21,7 @@ module ActiveRecord
       # Sets the primary key value.
       def id=(value)
         sync_with_transaction_state
-        write_attribute(self.class.primary_key, value) if self.class.primary_key
+        _write_attribute(self.class.primary_key, value) if self.class.primary_key
       end
 
       # Queries the primary key value.
diff --git a/activerecord/lib/active_record/attribute_methods/write.rb b/activerecord/lib/active_record/attribute_methods/write.rb
index 75c5a1a600..d7ddd35dff 100644
--- a/activerecord/lib/active_record/attribute_methods/write.rb
+++ b/activerecord/lib/active_record/attribute_methods/write.rb
@@ -17,7 +17,7 @@ module ActiveRecord
             generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
               def __temp__#{safe_name}=(value)
                 name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
-                write_attribute(name, value)
+                _write_attribute(name, value)
               end
               alias_method #{(name + '=').inspect}, :__temp__#{safe_name}=
               undef_method :__temp__#{safe_name}=
@@ -36,8 +36,7 @@ module ActiveRecord
         end
 
         name = self.class.primary_key if name == "id".freeze && self.class.primary_key
-        @attributes.write_from_user(name, value)
-        value
+        _write_attribute(name, value)
       end
 
       def raw_write_attribute(attr_name, value) # :nodoc:
@@ -46,10 +45,17 @@ module ActiveRecord
         value
       end
 
+      # This method exists to avoid the expensive primary_key check internally, without
+      # breaking compatibility with the write_attribute API
+      def _write_attribute(attr_name, value) # :nodoc:
+        @attributes.write_from_user(attr_name.to_s, value)
+        value
+      end
+
       private
         # Handle *= for method_missing.
         def attribute=(attribute_name, value)
-          write_attribute(attribute_name, value)
+          _write_attribute(attribute_name, value)
         end
     end
   end
diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 5776807507..1753322274 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -245,7 +245,7 @@ module ActiveRecord
       def ensure_proper_type
         klass = self.class
         if klass.finder_needs_type_condition?
-          write_attribute(klass.inheritance_column, klass.sti_name)
+          _write_attribute(klass.inheritance_column, klass.sti_name)
         end
       end
   end
diff --git a/activerecord/lib/active_record/timestamp.rb b/activerecord/lib/active_record/timestamp.rb
index 55f3a194a9..ae50bc6bc1 100644
--- a/activerecord/lib/active_record/timestamp.rb
+++ b/activerecord/lib/active_record/timestamp.rb
@@ -86,7 +86,7 @@ module ActiveRecord
 
         all_timestamp_attributes_in_model.each do |column|
           if !attribute_present?(column)
-            write_attribute(column, current_time)
+            _write_attribute(column, current_time)
           end
         end
       end
@@ -100,7 +100,7 @@ module ActiveRecord
 
         timestamp_attributes_for_update_in_model.each do |column|
           next if will_save_change_to_attribute?(column)
-          write_attribute(column, current_time)
+          _write_attribute(column, current_time)
         end
       end
       super(*args)