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
Wether by accident (the next_batch token doesn't get stored properly) or
by malicious intent (the server replays a message) an Olm encrypted to-device
message may appear multiple times.
This is usually fine since nothing bad happens, we don't decrypt the message
and the message gets thrown away.
Since the introduction of Olm session unwedging an undecryptable message
leads to the creation of a new fresh Olm session. To avoid this we
remember which Olm messages we already decrypted so they don't trigger
an unwedging dance.
