Remove direct calls of CellEndpoint#sendAndWait outside cells module

CellStub is used instead, which in many places leads to more compact code.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6742/