Migrated websocket service.

The websocket service is no longer provided by a different service,
controlled by a different thread running on another port.

Instead, it's now exposed directly over Flask routes, using
WSGI+eventlet+simple_websocket.

Also, the SSL context options have been removed from `backend.http`, for
sake of simplicity. If you want to enable SSL, you can serve Platypush
through a reverse proxy like nginx.

setup.py

@@ -63,6 +63,7 @@ setup(
         'alembic',
         'bcrypt',
         'croniter',
+        'eventlet',
         'flask',
         'frozendict',
         'gunicorn',
@@ -74,11 +75,12 @@ setup(
         'redis',
         'requests',
         'rsa',
+        'simple_websocket',
         'sqlalchemy',
         'tz',
         'websocket-client',
-        'websockets',
         'wheel',
+        'wsproto',
         'zeroconf>=0.27.0',
     ],
     extras_require={