Try putting room list handling behind a lock

Some of the logs relating to room list corruption appear to be badly timed race conditions so we'll try to linearize them here.

src/utils/MultiLock.ts

@@ -0,0 +1,30 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {EnhancedMap} from "./maps";
+import AwaitLock from "await-lock";
+
+export type DoneFn = () => void;
+
+export class MultiLock {
+    private locks = new EnhancedMap<string, AwaitLock>();
+
+    public async acquire(key: string): Promise<DoneFn> {
+        const lock = this.locks.getOrCreate(key, new AwaitLock());
+        await lock.acquireAsync();
+        return () => lock.release();
+    }
+}