- [Issue 1588] consolidate cache entry locking code:
  getEntry(lck) locking code logic is now consolidated in a single method 
  inside the EntryCache abstract class effectively moving it out of entry 
  cache implementing subclasses.

- [Issue 1589] entry cache implementations can lock and return stale entry: 
  Prevent stale entries by check [checking that given entry does indeed 
  exist in the cache], lock [acquiring a lock] and load [loading the entry].