Our main methods to get members nowadays ensure that the member list is
synchronized with the server. This is nice and convenient but might not
be desirable for a couple of reasons.
Firstly it might be costly to fetch all members at once depending on
what the client is doing and the number of rooms and secondly some
clients might have a hybrid setup where not everything is running on a
tokio thread, sending out requests is only possible on a tokio thread.
Use MxcUri instead of String for media URLs.
Fix wrong MXC URIs in tests.
Remove method parse_mxc no longer useful.
Apply new non-exhaustive types: CrossSigningKey, OneTimeKey and SignedKey.
Apply endpoint name change: send_state_event_for_key to send_state_event
