Rework XMPP Date/Time related code

- Fix "packet.Time is not thread-safe" (SMACK-543)
- Update packet.Time to XEP-0202

Add SDM.supportsFeature(), since this is a pattern that repeats over and
over again in Smack. Also add abstract Manager class, that takes care of
the weak reference to Connection, as this is also a repeating pattern in
Smack.

extensions/src/main/resources/org.jivesoftware.smackx/extensions.xml

@@ -10,5 +10,6 @@
         <className>org.jivesoftware.smackx.commands.AdHocCommandManager</className>
         <className>org.jivesoftware.smackx.ping.PingManager</className>
         <className>org.jivesoftware.smackx.privacy.PrivacyListManager</className>
+        <className>org.jivesoftware.smackx.time.EntityTimeManager</className>
     </startupClasses>
 </smack>