(core) revamp snapshot inventory

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

package.json

@@ -56,6 +56,7 @@
     "@gristlabs/pidusage": "2.0.17",
     "@gristlabs/sqlite3": "4.0.6-grist.1",
     "@popperjs/core": "2.3.3",
+    "async-mutex": "0.2.4",
     "axios": "0.18.0",
     "backbone": "1.3.3",
     "bootstrap": "3.3.5",