Lock the peer->mvipvt to avoid crashes with SIP history enabled

Lock the peer->mvipvt to avoid crashes with SIP history enabled

After the launch of 1.6 event-based MWI we have two threads handling the peer->mwipvt,
which cause issues with SIP history additions in combination with the max limit for
number of history entries.

Review: https://reviewboard.asterisk.org/r/1373/

(closes issue ASTERISK-18288)

Thanks to irrot for peer review. Work with this bug funded by IPvision AS

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@335319 65c4cc65-6c06-0410-ace0-fbb531ad65f3