Bug 9005: Allow user to disable syspref cache

Bug 9005: Allow user to disable syspref cache

Because C4::Context uses an in-memory hash for caching sysprefs,
changing a syspref under a multi-threaded persistent environment
requires a server restart. This patch makes it possible disable
the syspref cache.

To test:
1) If you are using a multi-threaded persistent server (Starman, etc.),
   change a syspref and note that the effects of the syspref change may
   or may not be visible on any given request before applying this patch.
   You will need to choose a syspref with obvious effects that can be
   seen by simply refreshing the page. I recommend enabling or
   disabling additional languages in the OPAC, since you can refresh the
   page a dozen times and reasonably expect to see the new behavior you
   set only 1/n of the time.
2) Apply patch.
3) Add "C4::Context->disable_syspref_cache();" to your koha.psgi file
4) Repeat step 1, noting that you never see the stale behavior.
5) Run test at t/db_dependent/Context.t.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
All tests passed.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>