Follow-up to r1567996:  Lost updates will be more common with the
global hit counters than the per-entry counters since the latter
are less busy.

That allows us to put a lower limit of 0 on the global counter
because in the long run, the sum of the entry hit counters will
always grow faster than the global hit value.

* subversion/libsvn_subr/cache-membuffer.c
  (): Explain the mechanics behind the race hit counters.
  (svn_membuffer_t): Switch back to an unsigned hit counter.
  (drop_entry,
   let_entry_age): Prevent global hit counter underflows.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568009 13f79535-47bb-0310-9956-ffa450edef68