spacemanager: Fix forwarding when disabled and add fast forward path

Addresses a bug in which PoolIoFileMessage was not forwarded to pool
manager when spacemanager was disabled.

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.

Note that read requests may still suffer from write congestion as
DoorTransferFinished must be intercepted by space manager and we
cannot distinguish between read and write requests without a
database lookup.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7902
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5886/