Backport some booking related commits from trunk.

Backport some booking related commits from trunk.

r17980: more transaction cleanups
--
r18502: Booking: make the circ module avoid booking code if the booking service
isn't up

Previously, it was not possible to turn the booking service off and have
working circ.  Now you should be able to turn it off if you don't need it.
--
r18514: If we already know we're not using booking, skip a redundant and
expensive test for the possibility of capturing a hold. We're just going to
try actually doing it a little farther down.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_1_6@18873 dcc99617-32d9-48b4-a31d-7c20da2025e4