Start time for jobs in the log output

Currently the log only shows elapsed times so we display [--:--:--] when we start a job. With only elapsed times, it's difficult to analyse which jobs overlapped which other ones - you can't tell in absolute terms when a job started and finished or how many are running at once.

I'd like to change this so START lines have the wallclock time attached to them, and the end of the job has the elapsed time with a '+' to indicate that it's relative to the start time, like this:

[13:35:51][85b53680][build:base/nano.bst                 ] START   Caching Artifact                                                                                                                  
[+00:00:03][85b53680][build:base/nano.bst                 ] SUCCESS Caching Artifact