Latch Gen2 cards before shutter closes

The ccdActors grab Gen2 cards just before readout (and the creation of the PHDU). But when we split CCD exposures to allow slews while reading out, that could happen after setup for the next field starts, and so some cards (esp. OBJECT) can be for the wrong field.

Way back when, Gen2 keys were supposed to be grabbed at the start of integration, but we never added the (fragile) logic to trigger on shutter opening, or from some new SPS command. Needs to be rethought.