documentation updates
removed remnants of db profiling
cleanup of entity caching code
removed query cache reset from unit test of acl. If this is needed, is bug in elgg core
introduces _elgg_invalidate_query_cache() to dry up the db query cache code
adds LRU cache
updated lru cache so easier to integrate
Fixes #4978 integrates LRU cache for db query cache
Merge pull request #5355 from cash/oom2

Fixes the DB query cache out of memory issues