feat: add onDisconnect callback to subscription

[krissvaa]

The onDisconnect callback is called on reconnect. This allows the Flux Connection to call the registered onDisconnect callbacks on reconnect. If the state is already RECONNECTING, subsequent calls to onReconnect will not trigger calling the onDisconnect callbacks multiple times.

Fixes: #398

Also as long-polling has issues with flux connections only websocket connection is supported. Long polling can still be enabled by passing atmosphere options from Connect.ts