Track sync server connectivity

[ikusteu]

Add the ability to track sync server connectivity:

• add onConnOpen and onConnClose to Transport interface
• extend WebSocketTransport to implement these methods

Current implementation:

• the client polls (pings the server) setting the timeout after which the connection is considered closed:
  • onConnClose is called
  • the socket is closed and connection retried at an interval
  • note: the transport is not considered closed at this point, it's merely retrying the connection (full close will stop it retrying)
• in case a sign of life from the server is received within the aforementioned timeout, the timeout is reset
• all server received messages are treated as a sign of life: both the explicit pong message, as well as any other server sent event

The timeout (how long we wait for sign of life) and interval (poll frequency) are configurable via pingTimeout and pingInterval transport options (respectively)