Something weird is happening with ElementX iOS. When
`SlidingSync::stop_sync` is called, the internal message `SyncLoopStop`
has time to be sent in the internal channel, but iOS decides to suspend
the code before the sync-loop processes it. When ElementX decides to
start the sync-loop again, it immediately processes the `SyncLoopStop`
message, and… stops the sync-loop.
This patch ensures that `SlidingSync::sync` drains the internal channel
before starting the sync-loop for real. `tokio::sync::mpsc::Receiver`
type has no `drain` method, so this patch implements its
own logic by calling `try_recv` in a loop, until it returns
`Err(TryRecvError::Empty)`.
Ivan Enderlin
9da1a2f48b