Bug 1115749 - Rename mIndexMonitor to mDemuxerMonitor. r=rillian
Bug 1115749 - Hold the demuxer monitor in MP4Reader::Seek. r=rillian

This method invokes Seek* on mDemuxer.
Bug 1115749 - Hold the demuxer lock while accessing metadata. r=rillian

We're invoking a number of methods on mDemuxer here.
Bug 1115749 - Assert that we hold the monitor at all the MP4Demuxer API entry points. r=rillian

MP4Demuxer accesses shared state (index, moofparser) in lots of shared helper
routines, so we should assume that it's not safe to touch the demuxer at all
without holding the lock (unless demonstrated otherwise). This stuff should
probably be redesigned, but that's a problem for another day.