With the code as it stands, calls to shutdown that happen before
serve_forever enters its loop will deadlock, and there's no simple way
for the user to avoid this. The attached patch prevents the deadlock and
allows multiple serve_forever..shutdown cycles, but it's pretty
complicated. I could make it a lot simpler by making shutdown permanent:
any later serve_forever calls would return immediately.

A third choice would be to add a .serve_in_thread function that returns
a token that can be used to shut down exactly that loop, instead of
putting .shutdown() on the server. Any opinions?