Improve refreshing annotations.
Improve refreshing annotations.

This improves refreshing annotations when a browser tab regains focus.
Before, any displayed annotations were refetched from the server.  Now,
only annotations that have changed are refetched.  If a displayed
annotation has been deleted, it is properly removed from the display.

DigitalSlideArchive/HistomicsTK,DigitalSlideArchive/HistomicsTK