Allow stopwatches to be restarted

It is quite useful to allow a stopwatch to be reset
or restarted in a single operation, instead of having
to handle this logic outside of the stopwatch object,
which is error-prone and unnecessary to require users
to reinvent.

Part of blueprint process-executor

Change-Id: Ie0304a4f1d5f6bf0460e9a134762d1403da56375
Merge "Allow stopwatches to be restarted"