Get rid of support for synchronous messages in CellNucleus

There is still a sendAndWait method in CellNucleus to support legacy
code, but it is implemented on top of the asynchronous logic.

sendAndWait is gone from CellAdapter and CellEndpoint. Higher level
code should use CellStub if synchronous behaviour is desired.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6758