Replace watchdog by asynchronous heartbeat messages #278
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The previous watchdog requests required waiting for a reply, which could slow down the system under load and also lead to false errors on slower systems (e.g. the CI). Because of this, this commit replaces the watchdog requests with asynchronous heartbeat messages, which don't require replies. Instead, we record the last heartbeat timestamp whenever we receive a heartbeat message. We then periodically check the time since the last received heartbeat message and consider the connection closed if too much time has passed.
This PR should fix the CI watchdog errors that we were seeing.