(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/Comm.ts

@@ -148,6 +148,11 @@ export class Comm extends EventEmitter {
   public async testServerShutdown() {
     if (this._wss) {
       for (const wssi of this._wss) {
+        // Terminate all clients. WebSocket.Server used to do it automatically in close() but no
+        // longer does (see https://github.com/websockets/ws/pull/1904#discussion_r668844565).
+        for (const ws of wssi.clients) {
+          ws.terminate();
+        }
         await fromCallback((cb) => wssi.close(cb));
       }
       this._wss = null;
@@ -259,7 +264,6 @@ export class Comm extends EventEmitter {
           await this._onWebSocketConnection(websocket, req);
         } catch (e) {
           log.error("Comm connection for %s threw exception: %s", req.url, e.message);
-          websocket.removeAllListeners();
           websocket.terminate();  // close() is inadequate when ws routed via loadbalancer
         }
       });