added an asyncio server example

[evvaaaa]

I can't get the async server to work. I think adding an example while troubleshooting this would be good.

Both READ_PV and WRITE_PV can be successfully set up and pvget'd, however I can't get the async handler to function. The pvput times out, and after exiting the server we get the error:

task: <Task pending name='Task-4' coro=<AttrWHandler.put() running at /.../p4p/example/asyncio_server.py:31> ... RuntimeWarning: coroutine 'AttrWHandler.put' was never awaited

I was judging this was the recommended way to set up a server from asynciotest.py?

[mdavidsaver]

asyncio code should not use Server.forever(), which is a convenience for threading code to park a main thread. It probably should have been added under Server.thread. I think it actually pre-dates my adding support for cothread/asyncio.

The doc. comment gives the basic template.

p4p/src/p4p/server/__init__.py

Lines 165 to 169 in 9040d4b

	with Server(*args, **kws):
	try;
	time.sleep(99999999)
	except KeyboardInterrupt:
	pass

A direct equivalent with asyncio would replace the blocking sleep.

             with Server(*args, **kws): 
                 await asyncio.sleep(99999999) 

Since KeyboardInterrupt doesn't work with asyncio, a more complete version would handle signals to allow for a graceful shutdown. (an example)

             with Server(*args, **kws): 
                 done = asyncio.Event()
                 loop.add_signal_handler(signal.SIGINT, done.set)
                 loop.add_signal_handler(signal.SIGTERM, done.set)
                 await done.wait()

[evvaaaa]

Ah okay that makes sense. The example works now.