Update websocket clients with rescan progress.

This change periodically (about every 10 seconds) notifies the
connected websocket client of the height of the last processed block
as part of the rescan.  This enables clients to listen for the
notification and keep track of how much progress a rescan has made
even without any results being found.  If the websocket connection is
lost during the rescan, on reconnect, clients may safely start over at
the last notified height.