[WFLY-3603] Add global resource notifications

Runtime attributes does not store their values in the resource models
and their read handler must be called before and after their write
handler to get the old and new value of the attribute.

JIRA: https://issues.jboss.org/browse/WFLY-3603
This is a nit, but you should be able to just use context.readResource(PathAddress.EMPTY_ADDRESS).getModel() as from this point the OSH should not be modifying the resource.

A simple readResource call will give you the current model used by the OC; once the OC clones the model (presumably in writeHandler.execute(...)) then thereafter a readResource call will return that clone.