use db notifiers

[adecaro]

The ttx finality view relies on the ttxdb to learn if a given transactions has been committed or rejected.
Periodically though, the view performs an explicit query to check the content of the db.
This query can be removed by using the notification mechanism provided by FSC implementing the interface

type Notifier interface {
	// Subscribe registers a listener for when a value is inserted/updated/deleted in the given table
	Subscribe(callback TriggerCallback) error
	// UnsubscribeAll removes all registered listeners for the given table
	UnsubscribeAll() error
}