Explicit change notifications

[groue]

This pull request addresses #1457 and adds a new API that helps applications deal with database changes that are not automatically detected by GRDB.

As a reminder, the changes that are not automatically detected are:

• Changes performed by external database connections.
• Changes performed by SQLite statements that are not both compiled and executed through GRDB APIs.
• Changes to the database schema, changes to internal system tables such as sqlite_master.
• Changes to WITHOUT ROWID tables.
• The deletion of duplicate rows triggered by ON CONFLICT REPLACE clauses (this last exception might change in a future release of SQLite).

Those changes are still undetected, but applications can explicitly notify them when appropriate, and trigger database observation tools (ValueObservation, DatabaseRegionObservation, and generally speaking all interested TransactionObserver):

try dbQueue.write { db in
    // Notify observers that some changes were performed in the database
    try db.notifyChanges(in: .fullDatabase)

    // Notify observers that some changes were performed in the player table
    try db.notifyChanges(in: Player.all())

    // Equivalent alternative
    try db.notifyChanges(in: Table("player"))
}