Restore cache removed in f2cc9a2

The failing test referenced doesn't fail with this removed, so the
failure was likely something else fixed during the work to make RTIR
3.0.0 work.

There are several places in RT::IR that assumed this cache existed and
would call RT::IR->CustomFields in tight loops.  Since it loads global
and queue-specific CFs in order to cache them, it makes looking up a
single CF expensive.

A private flush method was added in c3bd6477 but never actually used.
Start using it, since without it the CFs are indeed cached between
requests.  Luckily they're loaded by RT::SystemUser so there's no
leakages, other than what you might expect to happen if you test
visibility of CFs based on loading by the system user.
Clear HasConstituency cache

When 70eeabf4 added checking to only override methods if Constituency is
enabled, it added a cache.  Unfortunately, it never cleared the cache.
Combined with the CFs never clearing their cache (see f8568e7e2) this
surely led to some bizarre behavior if you disbaled the Constituency CF
that persisted until restarting apache.
Merge branch '3.2/caching'