(core) Regularly log data size in DocStorage.applyStoredActions using sqlite dbstat

Summary:
- Small cleanup: Make DocStorage implement OnDemandStorage, and remove unused execWithBackup
- Upgrade to new versions (.3) of @gristlabs/sqlite3 and connect-sqlite3 to use dbstat
- Add _logDataSize method which queries dbstat, adding up pgsize for tables loaded into the data engine
- Only complete _logDataSize every 5 minutes using new field _lastLoggedDataSize

Test Plan: Tested manually

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3277

package.json

@@ -76,11 +76,11 @@
   "dependencies": {
     "@googleapis/drive": "0.3.1",
     "@googleapis/oauth2": "0.2.0",
-    "@gristlabs/connect-sqlite3": "0.9.11-grist.1",
+    "@gristlabs/connect-sqlite3": "0.9.11-grist.4",
     "@gristlabs/express-session": "1.17.0",
     "@gristlabs/moment-guess": "1.2.4-grist.1",
     "@gristlabs/pidusage": "2.0.17",
-    "@gristlabs/sqlite3": "4.1.1-grist.1",
+    "@gristlabs/sqlite3": "4.1.1-grist.4",
     "@popperjs/core": "2.3.3",
     "accept-language-parser": "1.5.0",
     "async-mutex": "0.2.4",