An empty to-device request can happen if we're trying to re-share a
session with devices that are we're missing an olm session with so don't
send them out.
This patch removes the need to ask if a group session needs to be shared
it also adapts the method so it re-shares sessions if new users or
devices join the group.
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.
