Ticket 47750 - Entry cache part 2
Description: replacing PRLock for cache->c_mutex with PRMonitor.
Introducing cache_is_in_cache and replacing in_cache local variables
(e.g., e_in_cache) with the function call.
(cherry picked from commit 2c16d7d536481687b39f0d72c235aa8b7e7ef2d6)