Summary:
Deliberate changes:
* save snapshots to s3 prior to migrations.
* label migration snapshots in s3 metadata.
* avoid pruning migration snapshots for a month.
Opportunistic changes:
* Associate document timezone with snapshots, so pruning can respect timezones.
* Associate actionHash/Num with snapshots.
* Record time of last change in snapshots (rather than just s3 upload time, which could be a while later).
This ended up being a biggish change, because there was nowhere ideal to put tags (list of possibilities in diff).
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2646
