Fixed #28977 -- Changed local-memory cache to use LRU culling.

LRU culling turns every read into a kind of write to the cache: cache keys
are moved to the first position in the OrderedDict when they are retrieved.
The RWLock which permitted multiple readers while prioritizing a single
writer is obsolete since all accesses are now writes.

docs/releases/2.1.txt

@@ -108,7 +108,8 @@ Minor features
 Cache
 ~~~~~
 
-* ...
+* The :ref:`local-memory cache backend <local-memory-caching>` now uses a
+  least-recently-used (LRU) culling strategy rather than a pseudo-random one.
 
 CSRF
 ~~~~