Fixed processing of heartbeats and a session expiration and addeds support of aiohttp_cors

[Cykooz]

• Breaking change: Removed argument timeout from Session.__init__() and SessionManager.__init__().
• Breaking change: Argument heartbeat of SessionManager.__init__() renamed into heartbeat_delay.
• Breaking change: Session.registry renamed into Session.app.
• Breaking change: Deleted method SessionManager.route_url().
• Breaking change: Changed name of some routes.
• Breaking change: Dropped support of Python < 3.7
• Fixed processing of heartbeats and a session expiration.
• Fixed ping-pong based heartbeats for web-socket connections.
• Added arguments heartbeat_delay and disconnect_delay into Session.__init__().
• Added argument disconnect_delay into SessionManager.__init__().
• Added argument cors_config into function add_endpoint() to support of CORS settings from aiohttp_cors.
• Function add_endpoint() now returns all registered routes.
• Replaced returning instances of error HTTP responses on raising its as exceptions.

CI:

• TravisCI replaced by GItHub Actions

[Cykooz]

Somebody please remove Travis CI from pull request checks!