Websocket event manager

Websocket event manager is responsible for repeated events handling.

Each websocket request has in its structure the following field:

 "event": { "id": string, "date": timestamp } 

Event manager looks in its cache if the last request from a client device has the same event id:

• If it has - event manager returns result for previous event with this id, skipping server work;

• If it doesn't have - request is handled by the server;

• If event id is empty, event manager skips its check.

Event manager is implemented for request handling in case of network problems. Use different event ids for different requests!