[WFLY-292] Add caching for selected get operations
[WFLY-292] Validate cachable operations against last modified date
[WFLY-292] Add cache header to 304 response
[WFLY-292] Send complete model node for GET operation instead of just the result part
[WFLY-292] Adjust test cases for complete GET response
[WFLY-292] Switch cache validation from last modified date to ETag