Better base class for write-attribute handlers

ServerWriteAttributeOperationHandler isn't a very nice base for a write-attribute handler.

1) It does stuff in Stage.MODEL (validateResolvedValue()) that should be in Stage.RUNTIME.
2) It forces subclasses to register a Stage.RUNTIME handler, instead of providing that for them.
3) It doesn't provide very good support for handling rollbacks of runtime changes.
4) It's in the server module instead of controller.

I'm going to add a new AbstractWriteAttributeHandler in controller, deprecate ServerWriteAttributeOperationHandler, and convert all existing uses of it in the AS code base.