DM-22093: Store begin/end times of ap_pipe in ap_verify

[kfindeisen]

This PR adds a string completion timestamp to the metadata for all timing measurements. There are no test or documentation changes, as measurement metadata are provided "as-is" with no requirements on their availability or format.

Example output from inspect_jobs.py:

Measurements:
                      ap_pipe.ApPipeTime =  94.2989 s ({'estimator': 'pipe.base.timeMethod', 'end': '2019-11-16T00:12:02.754711'})
               pipe_tasks.ProcessCcdTime =   24.629 s ({'estimator': 'pipe.base.timeMethod', 'end': '2019-11-16T00:10:41.117107'})
                          ip_isr.IsrTime =   5.3994 s ({'estimator': 'pipe.base.timeMethod', 'end': '2019-11-16T00:10:20.938137'})
        pipe_tasks.CharacterizeImageTime =  10.0975 s ({'estimator': 'pipe.base.timeMethod', 'end': '2019-11-16T00:10:31.614739'})

[morriscb]

Looks simple and good enough for me. Thanks.