feat(dqlite): Add WatchModelMachines on machine WatchableService

[cderici]

This adds the following:

• Adds a WatchableService on top of the machine service, and implements the WatchModelMachines (old one is at state/watcher.go) on top of that WatchableService.
• Adds triggers for machine table (that puts uuid into the change).
• Return uuid from CreateMachine from the machine state layer
• Lifts up the uuid creation from the state layer up to the service layer (if we do retries we don't keep regenerating stuff).
• Renames WatchModelMachines -> WatchMachines and UpsertMachine -> CreateMachine

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• [ ] Integration tests, with comments saying what you're testing
• [ ] doc.go added or updated in changed packages

QA steps

 $ TEST_PACKAGES="./domain/schema/... -count=1 -race" make run-go-tests
 $ TEST_PACKAGES="./domain/machine/... -count=1 -race" make run-go-tests

Links

Jira card: JUJU-6197

[cderici]

/merge