LP#1514085 Vandelay in-database session tracking

LP#1514085 Vandelay in-database session tracking

Adds a new DB table vandelay.session_tracker for monitoring progress on
Vandelay enqueue and import sessions.

Enqueue and import APIs get a new option to exit early, returning the
newly created tracker object, so the caller can monitor the tracker
instead of listening to streamed responses, which are not supported in
browser client Dojo interfaces.

Teach the existing Dojo Vandelay UI to exit early on enqueu & export and
to poll for tracker data in lieu of waiting for streamed progress data.

On user merge / purge, trackers are migrated to the destination user.

Signed-off-by: Bill Erickson <berickxx@gmail.com>