(core) Add options to /status health-check endpoints to check DB and Redis liveness.

Summary:
- /status accepts new optional query parameters: db=1, redis=1, and timeout=<ms> (defaults to 10_000).
- These verify that the server can make trivial calls to DB/Redis, and that they return within the timeout.
- New HealthCheck tests simulates DB and Redis problems.
- Added resilience to Redis reconnects (helped by a test case that simulates disconnects)
- When closing Redis-based session store, disconnect from Redis (to avoid hanging tests)

Some associated test reorg:
- Move stripeTools out of test/nbrowser, and remove an unnecessary dependency,
  to avoid starting up browser for gen-server tests.
- Move TcpForwarder to its own file, to use in the new test.

Test Plan: Added a new HealthCheck test that simulates DB and Redis problems.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4054

test/gen-server/apiUtils.ts

@@ -51,9 +51,11 @@ export class TestServer {
     // TypeORM doesn't give us a very clean way to shut down the db connection,
     // and node-sqlite3 has become fussier about this, and in regular tests
     // we substitute sqlite for postgres.
-    for (let i = 0; i < 30; i++) {
-      if (!this.server.getNotifier().testPending) { break; }
-      await delay(100);
+    if (this.server.hasNotifier()) {
+      for (let i = 0; i < 30; i++) {
+        if (!this.server.getNotifier().testPending) { break; }
+        await delay(100);
+      }
     }
     await removeConnection();
   }