Support "seq_interval" qs param on _changes
Support "seq_interval" qs param on _changes

Supplying seq_interval=N will cause the changes feed to include a
sequence on every Nth row but set seq: null otherwise.  It's a
performance hack until we can deploy a more efficient format.

BugzID: 23637

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