Membuffer cache hit counter update done right:  introduce a separate
per-segment mutex for counter updates.  Since the critical section
is very tight, there is little actual lock contention and the sync.
overhead is virtually non-existent (tested with 100 concurrent threads
on one and two-socket systems).

* subversion/libsvn_subr/cache-membuffer.c
  (): Remove the comments about racy hit counter handling.
  (svn_membuffer_t): Mention that our profining counters are not
                     thread-safe and that we don't care.
  (drop_entry,
   let_entry_age): Since hit counter updates are now synchronized,
                   there can't be any underflows in the global counters
                   anymore.
  (svn_cache__membuffer_cache_create): Initialize the new mutex.
  (increment_hit_counters): Formally return an svn_error_code to make
                            this function usable with our mutex macro.
  (membuffer_cache_get_internal,
   membuffer_cache_has_key_internal,
   membuffer_cache_get_partial_internal,
   membuffer_cache_set_partial_internal): Synchronize hit counter updates.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568062 13f79535-47bb-0310-9956-ffa450edef68