Applications can now connect to multiple shards and switch between
their shards in an application. Note that the shard swapping is
still a manual process as this change does not include an API for
automatic shard swapping.

Usage:

Given the following configuration:

```yaml
production:
  primary:
    database: my_database
  primary_shard_one:
    database: my_database_shard_one
```

Connect to multiple shards:

```ruby
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to shards: {
    default: { writing: :primary },
    shard_one: { writing: :primary_shard_one }
  }
```

Swap between shards in your controller / model code:

```ruby
  ActiveRecord::Base.connected_to(shard: :shard_one) do
    # Read from shard one
  end
```

The horizontal sharding API also supports read replicas. See
guides for more details.

This PR also moves some no-doc'd methods into the private namespace as
they were unnecessarily public. We've updated some error messages and
documentation.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>