Wires need to be aware of the `scope_flag` when running on-the-fly scans

[kabanaty]

The on-the-fly sequence gathers additional data on the IOC side for motion engineers called "Scope Data". Scope data takes time to collect, and there is a SCOPEFLAG PV that indicates whether scope data is finished collecting. To avoid squashing incoming scope data with a new scan, on-the-fly scans should wait for a new scope_flag accessor to report ready before initiating another scan.