Fix memory leaks in stasis_channels and bridge_native_rtp

Fix memory leaks in stasis_channels and bridge_native_rtp

This patch fixes two memory leaks:
 * A memory leak in packing channels into a multi-channel blob payload when
   publishing dial messages. The multi-channel blob payload does not steal
   the references - this approach was chosen because it works well with the
   RAII_VAR macro. Unfortunately, this does mean that you actually have to use
   the RAII_VAR macro (or manually deref it yourself)
 * RTP instances returned as a result of one of the glue operations are ref
   counted and have to be de-ref'd appropriately. We now do that, as saying
   that we should do it and then not would be silly.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391479 65c4cc65-6c06-0410-ace0-fbb531ad65f3