There's no longer any need for the usnic BTL to have its own progress
thread: it can use the opal_progress_thread() infrastructure. This
commit removes the code to startup/shutdown the usnic-BTL-specific
progress thread and instead, just adds its events to the OPAL-wide
progress thread.
This necessitated a small change in the finalization step.
Previously, we would stop the progress thread and then tear down the
events. We can no longer stop the progress thread, and if we start
tearing down events, this will cause shutdown/hangups to be sent
across sockets, potentially firing some of the still-remaining events
while some (but not all) of the data structures have been torn down.
Chaos ensues.
Instead, queue up an event to tear down all the pending events. Since
the progress thread will only fire one event at a time, having a
teardown event means that it can tear down all the pending events
"atomically" and not have to worry that one of those events will get
fired in the middle of the teardown process.
bad508687e