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, 36 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 6397bc361a..00f4ee1aaa 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,39 @@
+*   Add basic API for connection switching to support multiple databases.
+
+    1) Adds a `connects_to` method for models to connect to multiple databases. Example:
+
+    ```
+    class AnimalsModel < ApplicationRecord
+      self.abstract_class = true
+
+      connects_to database: { writing: :animals_primary, reading: :animals_replica }
+    end
+
+    class Dog < AnimalsModel
+      # connected to both the animals_primary db for writing and the animals_replica for reading
+    end
+    ```
+
+    2) Adds a `connected_to` block method for switching connection roles or connecting to
+    a database that the model didn't connect to. Connecting to the database in this block is
+    useful when you have another defined connection, for example `slow_replica` that you don't
+    want to connect to by default but need in the console, or a specific code block.
+
+    ```
+    ActiveRecord::Base.connected_to(role: :reading) do
+      Dog.first # finds dog from replica connected to AnimalsBase
+      Book.first # doesn't have a reading connection, will raise an error
+    end
+    ```
+
+    ```
+    ActiveRecord::Base.connected_to(database: :slow_replica) do
+      SlowReplicaModel.first # if the db config has a slow_replica configuration this will be used to do the lookup, otherwise this will throw an exception
+    end
+    ```
+
+    *Eileen M. Uchitelle*
+
 *   Enum raises on invalid definition values
 
     When defining a Hash enum it can be easy to use [] instead of {}. This