(core) Manage memory used for websocket responses to reduce the risk of server crashes.

Summary:
- Implements MemoryPool for waiting on memory reservations.
- Uses MemoryPool to control memory used for stringifying JSON responses in Client.ts
- Limits total size of _missedMessages that may be queued for a particular client.
- Upgrades ws library, which may reduce memory usage, and allows pausing the websocket for testing.
  - The upgrade changed subtle behavior corners, requiring various fixes to code and tests.

- dos.ts:
  - Includes Paul's fixes and updates to the dos.ts script for manual stress-testing.
  - Logging tweaks, to avoid excessive dumps on uncaughtError, and include timestamps.

Test Plan:
- Includes a test that measures heap size, and fails without memory management.
- Includes a unittest for MemoryPool
- Some cleanup and additions to TestServer helper; in particular adds makeUserApi() helper used in multiple tests.
- Some fixes related to ws upgrade.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3974

app/client/components/GristWSConnection.ts

@@ -110,6 +110,7 @@ export class GristWSSettingsBrowser implements GristWSSettings {
 export class GristWSConnection extends Disposable {
   public useCount: number = 0;
   public on: BackboneEvents['on'];    // set by Backbone
+  public off: BackboneEvents['off'];    // set by Backbone
 
   protected trigger: BackboneEvents['trigger']; // set by Backbone