Refactor MatrixClient.encryptAndSendEvent (#4031)

* Replace `pendingEventEncryption` with a Set

We don't actually need the promise, so no need to save it.

This also fixes a resource leak, where we would leak a Promise and a HashMap
entry on each encrypted event.

* Convert `encryptEventIfNeeded` to async function

This means that it will always return a promise, so `encryptAndSendEvent` can't
tell if we are actually encrypting or not. Hence, also move the
`updatePendingEventStatus` into `encryptEventIfNeeded`.

* Simplify `encryptAndSendEvent`

Rewrite this as async.

* Factor out `MatrixClient.shouldEncryptEventForRoom`

* Inline a call to `isRoomEncrypted`

I want to deprecate this thing

spec/unit/queueToDevice.spec.ts

@@ -265,6 +265,7 @@ describe.each([[StoreType.Memory], [StoreType.IndexedDB]])("queueToDevice (%s st
         });
         const mockRoom = {
             updatePendingEvent: jest.fn(),
+            hasEncryptionStateEvent: jest.fn().mockReturnValue(false),
         } as unknown as Room;
         client.resendEvent(dummyEvent, mockRoom);