For some backends the process thread can change (e.g.
switch coreaudio headphone + internal speakers)
If there are existing x-thread event calls this can lead to
the following situation:
1) SessionEvent::operator new
2) audioengine process thread change
3) SessionEvent::operator delete -> crash, wrong thread
SessionEvent::operator delete can safely push the event back to
the pool for later cleanup..
fe85575a12