While working on some more indepth changes to Rails internal connection
management we noticed that it's confusing in some cases that the `role`
is implcit when using a `shard`. For example, if you passed a `shard`
and not a `role` in an un-nested block the default `role` would be
`writing`.

```
ActiveRecord::Base.connected_to(shard: :one) do
  # connected to writing
end
```

However in cases where nesting is used it could be confusing to
application authors that the role is inherited:

```
ActiveRecord::Base.connected_to(role: :reading) do
  ActiveRecord::Base.connected_to(shard: :one) do
    # will read from shard one replica, not write to primary
  end
end
```

Since this could be potentially confusing, and extremely hard to track
in complex applications, the best approach is to require `role` when
using `shard` which is what this PR does.

Note: the code for this method is...getting unweildy. Once the
`database` argument is fully deprecated we can remove most of the guards
and make `role` required by removing `nil` from the keyword argument.
Until then we need to support required arguments in this round about way.