Retry if GoogleStorage downloads fail and record how many times a digest could not be downloaded

BUG=skia:2930

Review URL: https://codereview.chromium.org/625863003
This breaks out the DB versioning code into a re-usable package within the shared "go" folder.

The perf specific code is moved back from the shared "go" package into perf.

This is in preparation for the next CL which will use the DB code in "golden".

BUG=skia:

Review URL: https://codereview.chromium.org/636873003