Fetch capabilities in the background (#4246)

* Fetch capabilities in the background

& keep them up to date

* Add missed await

* Replace some more runAllTimers

and round down the wait time for sanity

* Remove double comment

* Typo

* Add a method back that will fetch capabilities if they're not already there

* Add tests

* Catch exception here too

* Add test for room version code

spec/test-utils/client.ts

@@ -88,6 +88,6 @@ export const mockClientMethodsEvents = () => ({
 export const mockClientMethodsServer = (): Partial<Record<MethodLikeKeys<MatrixClient>, unknown>> => ({
     getIdentityServerUrl: jest.fn(),
     getHomeserverUrl: jest.fn(),
-    getCapabilities: jest.fn().mockReturnValue({}),
+    getCachedCapabilities: jest.fn().mockReturnValue({}),
     doesServerSupportUnstableFeature: jest.fn().mockResolvedValue(false),
 });