(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/server/lib/ITestingHooks.ts

@@ -9,6 +9,7 @@ export interface ITestingHooks {
   commShutdown(): Promise<void>;
   commRestart(): Promise<void>;
   commSetClientPersistence(ttlMs: number): Promise<number>;
+  commSetClientJsonMemoryLimit(newTotalSize: number): Promise<number>;
   closeDocs(): Promise<void>;
   setDocWorkerActivation(workerId: string, active: 'active'|'inactive'|'crash'): Promise<void>;
   flushAuthorizerCache(): Promise<void>;
@@ -17,4 +18,5 @@ export interface ITestingHooks {
   setActiveDocTimeout(seconds: number): Promise<number>;
   setDiscourseConnectVar(varName: string, value: string|null): Promise<string|null>;
   setWidgetRepositoryUrl(url: string): Promise<void>;
+  getMemoryUsage(): Promise<object>;  // actually NodeJS.MemoryUsage
 }