Store packet extensions in a HashMap

allowing O(1) lookups for PacketExtensions

The one EntityCapsManagerTest becomes obsolete with this change, as
duplicate extension elements (RFC 6120 § 8.4) are now no longer possible
after the stanza has been parsed (they still may be received on the
wire, but only the last duplicate will be added).

smack-extensions/src/test/java/org/jivesoftware/smackx/caps/EntityCapsManagerTest.java

@@ -69,12 +69,6 @@ public class EntityCapsManagerTest extends InitExtensions {
         assertTrue(di.containsDuplicateIdentities());
     }
 
-    @Test
-    public void testVerificationDuplicateDataForm() {
-        DiscoverInfo di = createMalformedDiscoverInfo();
-        assertTrue(EntityCapsManager.verifyPacketExtensions(di));
-    }
-
     private void testSimpleDirectoryCache(StringEncoder stringEncoder) throws IOException {
 
         EntityCapsPersistentCache cache = new SimpleDirectoryPersistentCache(createTempDirectory());