Retry and parallelize stats puller

[sethvargo]

This updates the stats-puller to retry the HTTP request 3 times. This will make us more resilient to temporary network blips. It also parallelizes pulling (default 5).

Release Note

Add client-side retry logic and parallelize stats puller. The default parallelize is 5, but it can be customized with `STATS_PULLER_MAX_WORKERS`. There is also a **behavior change**. The stats-puller previously always returned success (but logged errors on failure). This changes the puller to return a non-200 response code if there are still failures after all retries have executed.

[sethvargo]

https://github.com/google/exposure-notifications-verification-server/pull/1905/files?w=1 is a nicer review diff