spacemanager: Refactor message dispatching code

Ensures that message forwarding and reply logic is all places in
messageToForward/messageArrived, all error injection logic in processMessage,
and all other methods are ignorant to how and when messages are sent.

All improves error handling a bit.

The layering achieved by this class makes subsequent refactoring of the code
simpler.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6389/