Switch to per-exposure SuNSS exposure control

We started running SuNSS using the expected PFS exposure strategy: tell iic to start taking exposures, then tell iic to stop.

It turns out that we always gain by having the sunssActor control exposures one-by-one itself (we can control sunss tracking and account for telescope motion much more intelligently). So switch to that scheme.

Add a per-exposure thread and an object which applies logic to both the per-exposure timing and the telescope tracking/pointing/offsetting events.