DatabaseRegionObservation

[groue]

This PR continues the job of moving some RxGRDB features back to GRDB, so that we can share code between reactive engines (and even not use any reactive engine at all).

DatabaseRegionObservation is like ValueObservation, except that it does not notify fresh values on every change. Instead, it notifies all impactful transactions right after they have been committed, and before any other thread had any opportunity to perform further changes:

let observation = DatabaseRegionObservation(tracking: Player.all())
let observer = observation.start(in: dbQueue) { db: Database in
    print("Players were changed")
}

try dbQueue.write { db in
    try Player(name: "Arthur").insert(db)
}
// Prints "Players were changed"

DatabaseRegionObservation is not a tool that should be used frequently. The intended use cases are all pretty advanced:

• One needs to write in the database after an impactful transaction.

• One needs to synchronize the content of the database file with some external resources, like other files, or system sensors like CLRegion monitoring.

• On iOS, one needs to process a database transaction before the operating system had any opportunity to put the application in the suspended state.

• One want to build a database snapshot with a guaranteed snapshot content.