When a laptop is suspended or a phone is locked, the timer that is used
to check the liveness of the connection is paused and is not able to
detect that the heartbeat has failed.
Previously, emitting a message after resuming the page would lose the
message. The status of the timer will now be checked before sending the
message, so that it gets buffered and sent upon reconnection.
Note: we could also have used the Page Visibility API or a custom
setTimeout() method based on setInterval(), but this would not be as
reliable as the current solution.
Reference: https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
Related: https://github.com/socketio/socket.io/issues/5135
