Make every call log the digest they operate on.
Make every call log the digest they operate on.

- Try to log exactly once per request, which means a few more logging.debug()
  calls than desired.
- Logs both cache hit and misses.
- Downgrade methods when relevant.
- Remove unneeded comments.

It will be very helpful when diagnosing issues.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1838243002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py