enable feed=eventsource for /_db_updates
enable feed=eventsource for /_db_updates

The fix is adding `eventsource` to the allowed options to
/_db_updates?feed=eventsource, since the underlying implementation
is a global database’s _changes feed.

Closes COUCHDB-2665

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb