On the fsfs-format7 branch:  Add svn_cache__has_key() function
to our cache API.  Semantically, it is a stripped-down version
of svn_cache__get() but without actually copying and returning
any data.

* subversion/include/private/svn_cache.h
  (svn_cache__has_key): declare new API function

* cache.h
  (svn_cache__vtable_t): add has_key entry to cache vtable
* cache.c
  (svn_cache__has_key): implement new API function as forwarding
   to the actual cache implementation

* cache-inprocess.c
  (inprocess_cache_has_key_internal,
   inprocess_cache_has_key): implement has_key for in-process caches
  (inprocess_cache_vtable): update its vtable

* cache-memcache.c
  (memcache_has_key): implement has_key for memcached caches
  (memcache_vtable): update its vtable

* cache-membuffer.c
  (membuffer_cache_has_key_internal,
   membuffer_cache_has_key): implemente the has_key core functionality
  (svn_membuffer_cache_has_key,
   svn_membuffer_cache_has_key_synced): implement vtable functions
  (membuffer_cache_vtable,
   membuffer_cache_synced_vtable): update vtables


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1442051 13f79535-47bb-0310-9956-ffa450edef68