`open_with_db` also has the passphrase parameter without mentioning it
in its name. The old name also sounded like the passphrase was required
when it's actually optional.
As discussed with @poljar, the Matrix event we are creating is
`m.room.key`, so it's preferable to rename the function
`share_room_key`. Note: Olm calls the things an inbound group session,
that's where the name comes from, but it's not aligned with the
specification.
