func_periodic_hook: add module ref counting

This module lacked necessary module ref count incrementing and decrementing
when used.  This patch adds it.  There's already a datastore used, so doing the
ref counting along with the lifetime of the datastore provides a convenient
place to do it.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@412279 65c4cc65-6c06-0410-ace0-fbb531ad65f3