scheduler sends select_destinations notifications

scheduler_run_instance sends start/end notifications so you can tell how
long it takes for the scheduler to do its work.

The new select_destinations does not send any such notifications.

To make the new code patch more consistent with the notifications in the
old code patch, this change adds in start/end notifications.

The notification docs should be updated to reflect this change.

DocImpact

Closes-Bug: #1359835

Change-Id: I9b3f6619823c57cbc21d714edd78527c0015b4f6
Merge "scheduler sends select_destinations notifications"