Add check replication_active_slots.

[renard]

Check all replication slots and raise a warning is a slot is not active and hasn't been used or a critical alert if the slot has been used.

check_postgres_replication_active_slots --port=5432 --host=yellow

Specific named slots can be monitored using --include/--exclude.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>