Added support for acknowledgements to update cluster settings api

As a side note, the internal reroute call is now part of the ack mechanism. That means that if the response contains acknowledged flag, the internal reroute that was eventually issued was acknowledged too. Also, even if the request is not acknowledged, the reroute is issued before returning, which means that there is no need to manually call reroute afterwards to make sure the new settings are immediately applied.

Closes #3995