Layer synchronous CellStub use on top of async code

Rather than duplicating the same logic for synchronous and asynchronous
use cases, this patch changes CellStub to use the exact same asyncronous
behaviour in both cases. CellStub#sendAndWait is now a simple wrapper
around an asynchronous send with a blocking get on the Future.

Part of the superfluous synchronous code has been removed, simplifying
CellEndpoint and CellAdapter.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6740/