Log subprocess delays

Followup to !154 (merged). When subprocesses are delayed, waiting for other conflicting subprocesses to finish, they now log how long they were waiting.

We could extend this to record how long it spent waiting for each subprocess, but I think we get enough of an idea on that based on online monitoring.

Tests

I'm doing a 1000s run, with a 50s combine_stats (snapshot) interval, and a 51s calcfap interval. That should cause calcfap to interfere with itself, though from memory it's difficult to cause combine_stats to take a long time to run. I'll patch it to try to launch it twice in a row.