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
