Send heartbeat to server to avoid connection timeout

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Sending a heartbeat every 120 seconds should be sufficient for disconnect. This should be a configurable option as well.

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When the heartbeat interval is set (e.g. is greater than 0 seconds), the socket is open asynchronously/non-blocking, and thus runs in a free loop with a small delay. To completely close this issue, the loop should not be delayed at the bottom, on iterations where any message/data is received (the heartbeat timer can still be checked independently at the top).