isolate: make duration for expired file deletion sharding more granular
isolate: make duration for expired file deletion sharding more granular

Currently, deletion is distributed to shard per day, but query_expired
for a day seems reaching timeout before iterating all files in
specified duration.
http://shortn/_m7573SMMXe

Also that seems to cause lot of duplicate deletion request to GCS.
http://shortn/_uenSRbdnp1 shows many 404 for delete request to GCS.

To make sure files are deleted from oldest expiration_ts, let me make
sharding more granular in this CL.

Bug: 1004389
Change-Id: Ie2cdf1c5aa31ca4848bef82bb542a6ccf77a3012
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1916986
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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