Fixes "after_server_start" when using return_asyncio_server. (#1676)

* Fixes ability to trigger "after_server_start", "before_server_stop", "after_server_stop" server events when using app.create_server to start your own asyncio_server
See example file run_async_advanced for a full example

* Fix a missing method on AsyncServer that some tests need
Add a tiny bit more documentation in-code
Change name of AsyncServerCoro to AsyncioServer

examples/run_async_advanced.py

@@ -0,0 +1,38 @@
+from sanic import Sanic
+from sanic import response
+from signal import signal, SIGINT
+import asyncio
+import uvloop
+
+app = Sanic(__name__)
+
+@app.listener('after_server_start')
+async def after_start_test(app, loop):
+    print("Async Server Started!")
+
+@app.route("/")
+async def test(request):
+    return response.json({"answer": "42"})
+
+asyncio.set_event_loop(uvloop.new_event_loop())
+serv_coro = app.create_server(host="0.0.0.0", port=8000, return_asyncio_server=True)
+loop = asyncio.get_event_loop()
+serv_task = asyncio.ensure_future(serv_coro, loop=loop)
+signal(SIGINT, lambda s, f: loop.stop())
+server = loop.run_until_complete(serv_task)
+server.after_start()
+try:
+    loop.run_forever()
+except KeyboardInterrupt as e:
+    loop.stop()
+finally:
+    server.before_stop()
+
+    # Wait for server to close
+    close_task = server.close()
+    loop.run_until_complete(close_task)
+
+    # Complete all tasks on the loop
+    for connection in server.connections:
+        connection.close_if_idle()
+    server.after_stop()