(core) Starting to make webhooks more robust

Summary:
- Puts events on a queue in memory and ensures they are sent in the order they were generated.
- Makes the caller (Sharing.ts) wait until changed records have been fetched from the DB, but allows it to continue after while remaining work happens asynchronously.
- Gathers all new webhook events into an array so they can be backed up to the queue on redis in a single command (in a future diff).
- Uses changes in isReady to determine event type, no more 'existed before'

The structure of the code has changed a lot, so I think the scope of the diff needs to stop here. Lots of work is still deferred in TODOs.

Test Plan: Updated existing test. Actually dropped testing of retry on failures and slowness because it no longer made sense to keep that as part of the current test, so a new test for that will be added in a future diff.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3074

app/server/lib/Sharing.ts

@@ -18,7 +18,6 @@ import { ActionHistory, asActionGroup, getActionUndoInfo} from './ActionHistory'
 import {summarizeAction} from "./ActionSummary";
 import {ActiveDoc} from './ActiveDoc';
 import {makeExceptionalDocSession, OptDocSession} from './DocSession';
-import {TriggersHandler} from "./Triggers";
 import {WorkCoordinator} from './WorkCoordinator';
 
 // Describes the request to apply a UserActionBundle. It includes a Client (so that broadcast
@@ -284,7 +283,7 @@ export class Sharing {
       await this._activeDoc.processActionBundle(ownActionBundle);
 
       const actionSummary = summarizeAction(localActionBundle);
-      new TriggersHandler(this._activeDoc).handle(actionSummary);
+      await this._activeDoc.handleTriggers(actionSummary);
 
       // Broadcast the action to connected browsers.
       const actionGroup = asActionGroup(this._actionHistory, localActionBundle, {