Converted MultiTrackSelector from global to stream

Helgrind identified a race condition in MultiTrackSelector. The problem
was StringCutObjectSelector is not const thread safe (i.e. it has a const
function which modifies its internal state) and therefore only one thread
at a time can call it. Therefore MultiTrackSelector can not simultaneously
talk to multiple events. However, converting the module to a stream module
is thread safe.
Merge pull request #5324 from Dr15Jones/makeMultiTrackSelectorStreamModule

Converted MultiTrackSelector from global to stream