rgw: beast frontend closes connections on stop

rgw: beast frontend closes connections on stop

the strategy for stop relies on the fact that process_request() is
completely synchronous, so that io_context.stop() would still complete
each request and clean up properly

to tolerate an asynchronous process_request(), we instead need to drain
all outstanding work on the io_context so that io_context.run() can
return control natually to all of the worker threads. that would allow
us to suspend our coroutine in the middle of process_request(), and
still guarantee that process_request() will resume and run to completion
before the worker threads exit

each connected socket also counts as outstanding work, and needs to be
closed in order to drain the io_context. each connection now adds itself
to a connection list so that stop() can close its socket

Signed-off-by: Casey Bodley <cbodley@redhat.com>