Currently, there is no (simple) way to ask a model if it connects to a
single database or to multiple shards. Furthermore, without looping
through a model's connections, I don't believe there's an easy way to
return a list of shards a model can connect to.

This commit adds a `@shard_keys` ivar that's set whenever `.connects_to`
is called. It sets the ivar to the result of `shards.keys`. `shards` in
`.connects_to` defaults to an empty hash and therefore when calling
`connects_to database: {...}` `@shard_keys` will be set to an empty array.

`@shard_keys` is set _before_ the following lines:

```
if shards.empty?
  shards[:default] = database
end
```

This conditional sets the one and only shard (`:default`) to the value of `database`
that we pass to `.connects_to`. This allows for calling
`connected_to(shard: :default)` on models configured to only connect to
a database e.g.:

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

  connects_to database: { writing: :primary }
end

class UnshardedModel < UnshardedBase
end

UnshardedBase.connected_to(shard: :default) {
UnshardedBase.connection_pool.db_config.name } => primary
```

This is ultimately still an _unsharded_ model which is why `@shard_keys`
gets set before the conditional.

With the new `@shard_keys` ivar we need a way for descendants of the
abstract AR model to return that same value. For that we leverage the
existing `.connection_class_for_self` method. That method returns the
ancestor of the model where `.connects_to` was called, or returns self if
it's the connection class:

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

  connects_to database: { writing: :primary }
end

class UnshardedModel < UnshardedBase
end

ActiveRecord::Base.connection_class_for_self => ActiveRecord::Base

UnshardedBase.connection_class_for_self => UnshardedBase(abstract)

UnshardedModel.connection_class_for_self => UnshardedBase(abstract)
```

The new `.shard_keys` method is a getter which returns the value of
`@shard_keys` from the connection class or it returns an empty array.
The empty array is necessary in cases where `connects_to` was never
called.

Finally, I've added an `.connected_to_all_shards` method which takes all of the
arguments for `.connected_to` except for `shard`. Instead, it loops through
every shard key and then delegates everything else to `.connected_to`. I've
used `.map` instead of `.each` so that we can collect the results of each block.