This removes a massive performance issue since getting sessions is part
of every message send cycle.
Namely every time we check that all our devices inside a certain room
have established 1 to 1 Olm sessions we load the account from the store.
This means we go through an account unpickling phase which contains AES
decryption every time we load sessions.
Cache instead the account info that we're going to attach to sessions
when we initially save or load the account.
This modifies the cryptostore and storage logic in two ways:
* The cryptostore trait has only one main save method.
* The receive_sync method tries to save all the objects in one
`save_changes()` call.
This means that all the changes a sync makes get commited to the store
in one transaction, leaving us in a consistent state.
This also means that we can pass the Changes struct the receive sync
method collects to our caller if the caller wishes to store the room
state and crypto state changes in a single transaction.
