more inter-thread progress fixes; slight change of semantics:

more inter-thread progress fixes; slight change of semantics:

Background thread now *must* set "done" as last step. (they already do
since various error conditions don't result in "done")

This fixes a race: background thread Session::write_one_track() sets "done"
to true. Editor::freeze_route () continues, sets current_interthread_info
to NULL. thread continues and tries to set current_interthread_info.done
before terminating -> Crash.

This also ensures that singleton threads created with
"pthread_create_and_store" remain unique.