spacemanager: Postpone DB update on pool selection

Addresses issues in automatic space reservation for non-srm transfers.

Space manager intercepts both the select pool request and the request pool
reply. In the existing code, space manager selects a link group and creates or
updates a space reservation in the select pool request. If pool selection fails
in pool manager, space manager uses the intercepted reply message to attempt to
rollback what it did when intercepting the request.

The existing code was suboptimal for several reasons. First, the rollback done
when pool selection failed is incomplete, as it only works for srm managed
reservations (see the ticket). Second, we could just as well wait with the DB
update until we get the successfull reply rather than attempting to role back
the changes.

This patch changes the code such that the DB isn't updated until the reply
is received.

Target: trunk
Require-notes: yes
Require-book: no
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8163
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6352/