cells: Get rid of thread local holding the current message

The same functionality can be achieved by returning a Reply from message
handlers. The Reply functionality had to be adjusted slightly to cover an
interesting usecase in multicaster.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7513/