Optimize ValidateModelStepHandler

ValidateModelStepHandler is taking about 100ms during boot. We should optimize this.

Some thoughts:

1) Just add 1 step for it with all needed addresses, rather than 1 step per address.
2) Doing 1) makes it possible to cache looked up resources and MRRs throughout execution so we don't need to do context.readResource[Registration]FromRoot multiple times for the same address.
3) Add a Map<String, AttributeAccess> getAttributeAccesses() to MRR so we can get all of them for an address in one go and avoid having to continually cycle through the MRR tree.