Optimize memory usage for SimplePrivilegeCache

The implementation of SimplePrivilegeCache and in particular the 2 map<String, Set<String>> fields, cachedAuthzPrivileges and wildCardAuthz is not memory efficient. The strings stored in these structures can be quite long e.g. (server=server1->database=b1, (server=server1->database=b1->action=insert)). For really big installations with lots of entities (roles and privileges) these fields may end up taking some non-substantial amount of memory.

We should come up with of a more efficient memory representation to implement PrivilegeCache