[CLIENT] Remove unnecessary intermediate interfaces

Client, ClusterAdminClient and IndicesAdminClient had corresponding
intermediate `internal` interfaces that are unnecessary and cause
a lot of casting. This commit removes the intermediate interfaces
and uses the super interfaces directly.

This commit also adds Releaseable to `Node` and `Client` in order to
be used with utilities like try / with.

Closes #4355
Closes #6517
Remove unused executor() method from FilterClient