srm: Terminate db threads on shutdown

The patch ensures that db related threads are terminated when the SRM
shuts down. This both addresses warnings during dCache shutdown about
unterminated threads, and it addresses attempted database accesses
after the connection pool has been shut down.

The database garbage collector threads have been replaced by a single
scheduled executor. It is single threaded to reduce the load the
low priority GC activity puts on the database.

The thread pools have been configured to use named threads.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6788/