Add the ability to specify that a frame should not be considered for caching

for uses in cases where you *know* that it will do no good.  This patch was
inspired by file for use in some work of his on mixmonitor/chanspy.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@41958 65c4cc65-6c06-0410-ace0-fbb531ad65f3