[videodb] move AnnounceUpdate|Remove to statics
[announces] add 'transaction' attribution to OnUpdate, OnRemove if scanning.
drop unneeded tracking of database updating, using the 'transaction' attribute of OnUpdate/OnRemove instead
Merge pull request #4298 from jmarshallnz/scan_notify

RFC: Scan notify - add "scanning" parameter to OnUpdate, OnRemove announcements