(core) add missing tsconfig file that affects IDEs

Summary:
I missed committing a file that is important for editing files comfortably in the ext directory in an IDE. This diff:

 * Adds tsconfig-base-ext.json - that was the only intended change
 * Unrelated: Forces all creation of connections to the home db through a new `getOrCreateConnection` method which changes the `busy_timeout` if using Sqlite. This was an attempt to fix random "database is locked" test failures. I believe multiple connections to the home db as an sqlite file do not happen in self-hosted Grist (where there is a single node process) or in our SaaS (where the database is in postgres). It does affect Grist started using `devServerMain.ts` (where multiple processes accessing same database are started) or various test configurations when extra database connections are opened.
 * Unrelated: I added a `busy_timeout` for session storage, when it uses Sqlite. Again, I don't believe this affects self-hosted Grist or our SaaS.
 * Tweaked a `BillingDiscount` test that looked perhaps vulnerable to a stripe request stalling.

I can't be sure my tweaks actually help, since I didn't succeed in replicating the failures. Update: looks like the "locked" error can still happen :(

Test Plan: manual

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3450

app/gen-server/lib/HomeDBManager.ts

@@ -50,6 +50,7 @@ import {
   now,
   readJson
 } from 'app/gen-server/sqlUtils';
+import {getOrCreateConnection} from 'app/server/lib/dbUtils';
 import {makeId} from 'app/server/lib/idUtils';
 import * as log from 'app/server/lib/log';
 import {Permit} from 'app/server/lib/Permit';
@@ -60,10 +61,8 @@ import {Request} from "express";
 import {
   Brackets,
   Connection,
-  createConnection,
   DatabaseType,
   EntityManager,
-  getConnection,
   SelectQueryBuilder,
   WhereExpression
 } from "typeorm";
@@ -345,14 +344,7 @@ export class HomeDBManager extends EventEmitter {
   }
 
   public async connect(): Promise<void> {
-    try {
-      // If multiple servers are started within the same process, we
-      // share the database connection.  This saves locking trouble
-      // with Sqlite.
-      this._connection = getConnection();
-    } catch (e) {
-      this._connection = await createConnection();
-    }
+    this._connection = await getOrCreateConnection();
     this._dbType = this._connection.driver.options.type;
   }