Record true start/end time for build steps

[catlee]

Buildbot currently records the start/end time for steps on the master. This means that end time of step N is always the same as the start time of step N+1.

When master load is high, the latency between a slave actually finishing a command and receiving the next one can be significant.

One idea is to have the slaves record the command start time internally, and then when the command is done, send the slave start/end time to the master. The master would use its version of the start time as the step start time, and then calculate the delta between slave start time as master start time as clock scew/network latency and add it to the slave's end time.

[dustin]

This would make ETA calculations a good bit more accurate, too.