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 which means it can not be a global module. However, 
converting the module to a stream module is thread safe.