Decouple WebSocket implementation from DynamoDB

[jordaneremieff]

The websockets implementation needs better test coverage, error handling, documentation, etc.

[jordaneremieff]

So I finally got around to working out a better WebSocket implementation, and I'm thinking it is probably unnecessary to require DynamoDB to use it. All we really need is the connection ID of the client in APIGateway and then either Mangum itself could provide a configurable backend class, or leave it completely up to application to implement storage.

[jordaneremieff]

Closed by #99