lock.  The lock is now never held for a long time, only while
self.thread is being assigned or inspected, and while a thread is
being created, started or stopped.  Waiting for an event or joining a
thread is now done without holding the lock.  There is no longer a
fear of AttributeError.

When a thread join doesn't succeed within 30 seconds, a message is
logged, and depending on the situation, the join is retried.