Basic API for connection switching

This PR adds the ability to 1) connect to multiple databases in a model,
and 2) switch between those connections using a block.

To connect a model to a set of databases for writing and reading use
the following API. This API supercedes `establish_connection`. The
`writing` and `reading` keys represent handler / role names and
`animals` and `animals_replica` represents the database key to look up
the configuration hash from.

```
class AnimalsBase < ApplicationRecord
  connects_to database: { writing: :animals, reading: :animals_replica }
end
```

Inside the application - outside the model declaration - we can switch
connections with a block call to `connected_to`.

If we want to connect to a db that isn't default (ie readonly_slow) we
can connect like this:

Outside the model we may want to connect to a new database (one that is
not in the default writing/reading set) - for example a slow replica for
making slow queries. To do this we have the `connected_to` method that
takes a `database` hash that matches the signature of `connects_to`. The
`connected_to` method also takes a block.

```
AcitveRecord::Base.connected_to(database: { slow_readonly: :primary_replica_slow }) do
  ModelInPrimary.do_something_thats_slow
end
```

For models that are already loaded and connections that are already
connected, `connected_to` doesn't need to pass in a `database` because
you may want to run queries against multiple databases using a specific
role/handler.

In this case `connected_to` can take a `role` and use that to swap on
the connection passed. This simplies queries - and matches how we do it
in GitHub. Once you're connected to the database you don't need to
re-connect, we assume the connection is in the pool and simply pass the
handler we'd like to swap on.

```
ActiveRecord::Base.connected_to(role: :reading) do
  Dog.read_something_from_dog
  ModelInPrimary.do_something_from_model_in_primary
end
```

1 files changed, 3 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 392602bc0f..b53681ee20 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -124,6 +124,8 @@ module ActiveRecord
 
       mattr_accessor :belongs_to_required_by_default, instance_accessor: false
 
+      mattr_accessor :connection_handlers, instance_accessor: false, default: {}
+
       class_attribute :default_connection_handler, instance_writer: false
 
       self.filter_attributes = []
@@ -137,6 +139,7 @@ module ActiveRecord
       end
 
       self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new
+      self.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
     end
 
     module ClassMethods