add the ability to stop service when database relations gone

[delgod]

Issue

There no specific event in the requires library to handle the event of a relation being removed. Application charms can observer the relation departed/broken events, but that won't handle (or at least it would need more code to handle) all situations like, for example, aliased relations.

Solution

• Create new relation departed events (DatabaseDepartedEvent) to be triggered when a database unit departs from the relation.

Context

• Custom logic is needed in DatabaseDepartedEvent to correctly serialize and deserialize the departing unit information.
• Aliased events are also emitted.

Testing

• A new unit test was created (test_on_relation_departed).

Release Notes

• Add specific events to handle database units departing from the relation.

[delgod]

The "database_requires" and "database_provides" libraries are now deprecated in favour of "data_interfaces", which should provide a more comprehensive framework to extend to other technologies beside databases. On this migration, the signature of "DatabaseRequires" and "DatabaseProvides" did not change. PR should be created for a new library.