dispatcher: log duration of dispatcher call

Yes, we anyway log the timestamps for every log message. So one could
always calculate the offset. However, when you read a logfile, it can be
cumbersome to stop looking at where you currently are to find the
start/end of a call. For convenience, log the duration explicitly.

!1251
58 jobs for th/dispatch-log-timings in 31 minutes and 12 seconds (queued for 14 seconds)