(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-ti.ts

@@ -13,6 +13,7 @@ export const ITestingHooks = t.iface([], {
   "commShutdown": t.func("void"),
   "commRestart": t.func("void"),
   "commSetClientPersistence": t.func("number", t.param("ttlMs", "number")),
+  "commSetClientJsonMemoryLimit": t.func("number", t.param("newTotalSize", "number")),
   "closeDocs": t.func("void"),
   "setDocWorkerActivation": t.func("void", t.param("workerId", "string"), t.param("active", t.union(t.lit('active'), t.lit('inactive'), t.lit('crash')))),
   "flushAuthorizerCache": t.func("void"),
@@ -21,6 +22,7 @@ export const ITestingHooks = t.iface([], {
   "setActiveDocTimeout": t.func("number", t.param("seconds", "number")),
   "setDiscourseConnectVar": t.func(t.union("string", "null"), t.param("varName", "string"), t.param("value", t.union("string", "null"))),
   "setWidgetRepositoryUrl": t.func("void", t.param("url", "string")),
+  "getMemoryUsage": t.func("object"),
 });
 
 const exportedTypeSuite: t.ITypeSuite = {