don't check .metadata folder for tabs and nocommits
[TEST] Use test.bwc.version if compatibility version is not present
[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
Changed the type of field docCounts to IntArray instead of LongArray, because a shard can't hold more than Integer.MAX_VALUE a LongArray just takes unnecessary space.

Closes #6529