Add a housekeeping procedure for optimized acls

NXP-4557 has a little drawback: it does not clean the unused read acls from the read_acls.

This may decrease the performance in a long run and requires an housekeeping procedure.

Note that the executing a "SELECT nx_rebuild_read_acls();" will do the job but it will take time on large scale storage.

There must be something lighter like a nx_vacuum_read_acls().

This procedure should be exposed using JMX so it can be easily scheduled.