Bug 30728: Only trigger real-time holds queue update if enabled

Bug 30728: Only trigger real-time holds queue update if enabled

This patch makes the places in which Koha enqueues holds queue for
real time updates verify the feature is enabled.

To test:
1. Apply this patches up to the unit tests
2. Run:
   $ updatedatabase
   $ kshell
  k$ git diff origin/master --name-only | grep -e '\.t$' | xargs prove
=> FAIL: tests fail, the code doesn't care about the syspref
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Be happy!
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>