(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/server/lib/ICreate.ts

@@ -21,11 +21,20 @@ export interface ICreate {
   Billing(dbManager: HomeDBManager): IBilling;
   Notifier(dbManager: HomeDBManager, gristConfig: GristLoadConfig): INotifier;
   Shell(): IShell|undefined;
-  ExternalStorage(bucket: string, prefix: string): ExternalStorage|undefined;
+
+  // Create a space to store files externally, for storing either:
+  //  - documents. This store should be versioned, and can be eventually consistent.
+  //  - meta. This store need not be versioned, and can be eventually consistent.
+  // For test purposes an extra prefix may be supplied.  Stores with different prefixes
+  // should not interfere with each other.
+  ExternalStorage(purpose: 'doc' | 'meta', testExtraPrefix: string): ExternalStorage|undefined;
+
   ActiveDoc(docManager: DocManager, docName: string): ActiveDoc;
   DocManager(storageManager: IDocStorageManager, pluginManager: PluginManager,
              homeDbManager: HomeDBManager|null, gristServer: GristServer): DocManager;
   NSandbox(options: ISandboxCreationOptions): ISandbox;
 
   sessionSecret(): string;
+  // Get configuration information to show at start-up.
+  configurationOptions(): {[key: string]: any};
 }