fix “no per-thread pool” abort

fix “no per-thread pool” abort

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..