Add single mechanism for sub-actors to get gen2 visits

In operation, the iicActor requests visits from the gen2Actor and distributes them to lower-level actors. The iicActor will be the only one which does this.

For engineering, low-level actors (ccdActor, mcsActor, hxActor, spsActor, spsaitActor, etc) need to be able to get their own visits. A single mechanism should be shared by all. Probably in tron_actorcore.