This patch changes `matrix_sdk::Client::sliding_sync_proxy` to be a
`std::sync::RwLock<Option<Url>>` instead of `tokio::sync::RwLock<_>`.
It means that all methods reading or writing this field are sync instead
of async, which makes the code a lot more easier. Having an async-aware
lock wasn't necessary here.
Ivan Enderlin
9bcc50fe2f