use fetchTimestamp efficiently

[aappling-usgs]

we came pretty close with vizlab 0.2.2.9000 and hurricane-maria, but there were these remaining challenges:

• we could have used a finer resolution for checking the timestamps of the precip shapefiles in the fetch stage (Handle time stamps on precip data. DOI-USGS/hurricane-maria#13)

• we didn't fully capture the multi-file / folder dependency for processing precip shapefiles (process.compile_precip wasn't rebuilding when the folder dependency changed DOI-USGS/hurricane-maria#28)

• something about exceededTimeToLive wasn't working quite right (lag in exceededTimeToLive builds vizlab#229; this is probably a vizlab issue, so in vizstorm just be vigilant for opportunities to diagnose/correct/have patience with the problem)

• we should also take a final look at this template after the dust has settled on other issues, to make sure we're still using fetchTimestamp efficiently at that point

[jordansread]

Probably want to update with USGS-VIZLAB/vizlab#316