WL#4322: Improve memory handling for grant/acl/servers/plugins tables

Frequent updates to grant tables, including servers table and plugin, via 
appropriate statements (e.g. GRANT/REVOKE, CREATE/DROP USER, CREATE/DROP 
SERVER, INSTALL/UNINSTALL plugin) may increase memory usage.

As not-freed memory chunks are relatively small, one would experience noticable 
memory increase only if there were thousands or millions updates to grant 
tables.

This happens because when removing a record (REVOKE/DROP/UNINSTALL) from cache, 
we do not release memory back to an operating system. This memory is freed on 
FLUSH PRIVILEGES or server restart.

To avoid memory usage increase, we should release memory immediately.