Explore pthread_once and friends for initialization

[gdamore]

We currently use our own locks, which are a bit dodgy, and possibly less efficient.

We could conceivably use pthread_once or similar functions which should be highly optimized for the various platforms.

Note that restarting after nng_fini might not be supported, or we might need to separate functions that perform once-ever initialization from those that might need to be called after finalization (e.g. for starting up worker threads after they have been shut down.)