poolmanager: Get rid of ThreadPool interface

Use a regular Executor instead. The following properties are now
obsolete:

threadPool
poolmanager.plugins.thread-pool

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6785/