(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

app/common/DocSnapshot.ts

@@ -0,0 +1,37 @@
+/**
+ * Core metadata about a single document version.
+ */
+export interface ObjSnapshot {
+  lastModified: string;
+  snapshotId: string;
+}
+
+/**
+ * Extended Grist metadata about a single document version.  Names of fields are kept
+ * short since there is a tight limit on total metadata size in S3.
+ */
+export interface ObjMetadata {
+  t?: string;     // timestamp
+  tz?: string;    // timezone
+  h?: string;     // actionHash
+  n?: number;     // actionNum
+  label?: string;
+}
+
+export interface ObjSnapshotWithMetadata extends ObjSnapshot {
+  metadata?: ObjMetadata;
+}
+
+/**
+ * Information about a single document snapshot in S3, including a Grist docId.
+ */
+export interface DocSnapshot extends ObjSnapshotWithMetadata {
+  docId: string;
+}
+
+/**
+ * A collection of document snapshots.  Most recent snapshots first.
+ */
+export interface DocSnapshots {
+  snapshots: DocSnapshot[];
+}