cells: Further refine LoginManager startup and shutdown

Addresses flaws in server socket recreation in case of failures,
makes the code more DRY, uses thread pool for rejected connections,
kills children early, and waits for them to die before terminating
the login cell factory, removes legacy socket shutdown hack.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6326/