TRUNK-2671 Cache handlers in HandlerUtil

(cherry picked from commit cc3748cf53b5415a4ffd7d8a8a5060c4baf41fc6)

Conflicts:
	api/src/main/java/org/openmrs/util/OpenmrsConstants.java
	api/src/test/java/org/openmrs/aop/RequiredDataAdviceTest.java