box: don't destroy latch/fiber_cond that may have waiters at exit

fiber_cond_destroy() and latch_destroy() are no-op on release builds
while on debug builds they check that there is no fibers waiting on
the destroyed object. This results in the following assertion failures
occasionally hit by some tests:

  src/latch.h:81: latch_destroy: Assertion `l->owner == NULL' failed.
  src/fiber_cond.c:49: fiber_cond_destroy: Assertion `rlist_empty(&c->waiters)' failed.

We can't do anything about that, because the event loop isn't running at
exit and hence we can't stop those fibers. So let's not "destroy" those
global objects that may have waiters at exit, namely

  gc.latch
  ro_cond
  replicaset.applier.cond