Breaking changes for dependent charms:
- `self.db: str` has been removed in favour of
`self.dbs: Mapping[str, str]` mapping relation names to dabatase names
- interface name for database has changed
from mysql_datastore to mysql_client.
- The default database relation name has changed from `shared-db`
to `database`. We'll follow this convention in all dependent charms.
Change-Id: I750a8311c1e3db0b414207f712fa9061004b6920
7684a0db73