Adjust per-logsource sleep time for real (wall-clock) time already spent

This patch tracks the wall-clock time when we first see log timestamps (epochs) per-source, in order to reduce the inter-log delay by real-time already spent, and to not add further delays when multi-source logs have evidence of buffering-delays.  As a result, now replaying 24 hours of a contiguous multi-source log file gets much closer to 100% done in 24 hours.