ftp: Use shared thread pool for various processing threads

This patch makes the ftp door use the shared thread pool for
the I/O thread, for the message processing thread, and the
the callback thread.

The patch also updates AbstractCell to use the Cell's message
thread. This saves one additional thread instantiation for
each cell instance.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6331/