srm: Get rid of ThreadManager

SRM is updated to use a regular Executor for message processing. Introduces the
following new configuration properties:

srm.cell.limits.message.threads.min
srm.cell.limits.message.threads.max
srm.cell.limits.message.threads.max-idle-time
srm.cell.limits.message.threads.max-idle-time.unit
srm.cell.limits.message.queue.max

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6784/