Listeners for main server process (#2018)

* Initial POC

* Add test case

* Resolve create_server and Gunicorn serve)

* add testr coverage

sanic/worker.py

@@ -7,6 +7,7 @@ import traceback
 
 from gunicorn.workers import base as base  # type: ignore
 
+from sanic.log import logger
 from sanic.server import HttpProtocol, Signal, serve, trigger_events
 from sanic.websocket import WebSocketProtocol
 
@@ -72,6 +73,15 @@ class GunicornWorker(base.Worker):
         )
         self._server_settings["before_start"] = ()
 
+        main_start = self._server_settings.pop("main_start", None)
+        main_stop = self._server_settings.pop("main_stop", None)
+
+        if main_start or main_stop:  # noqa
+            logger.warning(
+                "Listener events for the main process are not available "
+                "with GunicornWorker"
+            )
+
         self._runner = asyncio.ensure_future(self._run(), loop=self.loop)
         try:
             self.loop.run_until_complete(self._runner)