log-gatherer: track last-heard-from time

[Brian Warner]

It would be useful if the log-gatherer were to keep track of the last time we've heard anything from a given supervised process. This could be last-log-message-received and/or connection-level keepalive timestamp.

[Brian Warner]

It would also be useful for the log-gatherer to keep track of the divergence between the emit-time and receipt-time for each of its children: if the clocks are somewhat synchronized, then the divergence is an indication of how large/backed-up the sender's log-sending queue is.