Service: only emit changed signals when values change
This change adds a check/filter to the writer service such that it only emits changed notifications for values in the database who's values changed as a result of the write.
Since the check is done in the central writer service which stores the canonical copy of the database, there is no opportunity for race conditions.
The issues caused by this bug are similar to !1 (merged)
See existing discussion on bugzilla here: https://bugzilla.gnome.org/show_bug.cgi?id=789639