Clean up MWI topic pool before message type destruction

Topics need to be disposed of prior to the message types that are published
on them. This includes topic pools. This prevents an assertion from being
raised on shutdown.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391040 65c4cc65-6c06-0410-ace0-fbb531ad65f3