Allow setting initial heartbeat interval

[BinaryFissionGames]

For agent/server heartbeats, the agent initially starts with a 30 second heartbeat interval, which can then be updated by the server if it chooses to.

However, there doesn't seem to be a way to update the initial interval through the client or in the start settings, which means that unless the server updates the interval, the agent is stuck at this 30 second interval.

For us, this is a problem because we want to disable heartbeats (set the interval to 0) unless the server explicitly requests them by setting the interval, for backwards compatibility reasons.