Update Atom syndication

[ddnebert]

GeoNetwork catalog should expose a queryable Atom endpoint that includes simple entries that include: title, identifier, date-time of change, type of change (add, update, delete). This would support those interested in subscribing to catalog changes and to support multi-catalog synchronization. The identifier could be used by other operations (like GetRecordById) to fetch or present records mentioned in the atom feed. If applied through OpenSearch, a query could customize the feed content by including one or more query terms for date range, change type, or general text search.

[fxp]

See discussion : ​http://osgeo-org.1803224.n2.nabble.com/Proposal-for-vote-Metadata-Notifications-to-Remote-Targets-td5642234.html#a5647087