[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
To be more safe, please use

ModelNode newValue = model.has(attributeName) ? model.get(attributeName) : new ModelNode();

The "get" call will add the node if it doesn't exist, and this code should not do that.