Fix/2.2/rb5886

Addresses a performance issue in which spacemanager congestion caused
by heavy write activity slows down read requests, even though read
requests are simply forwarded by pool manager. The issue is that the
forwarding of messages related to a read request happens on a thread
pool (manager by ThreadManager) that is also used by write requests.
This patch addresses the problem by forwarding such message immediately
rather than scheduling them on a different thread pool.