OF-984 Avoid using interned domains as sync keys

A few places all use the domain.intern() as the key for a synchronized block.

This can lead to clashes when multiple semantics conflict, and this can in turn
lead to a deadlock, as witnessed in OF-984.

This changes two cases to use a prefixed string to namespace the locking.

src/java/org/jivesoftware/openfire/session/LocalOutgoingServerSession.java

@@ -592,7 +592,7 @@ public class LocalOutgoingServerSession extends LocalServerSession implements Ou
         String senderDomain = packet.getFrom().getDomain();
         boolean processed = true;
         if (!getAuthenticatedDomains().contains(senderDomain)) {
-            synchronized (senderDomain.intern()) {
+            synchronized (("Auth::" + senderDomain).intern()) {
                 if (!getAuthenticatedDomains().contains(senderDomain) &&
                         !authenticateSubdomain(senderDomain, packet.getTo().getDomain())) {
                     // Return error since sender domain was not validated by remote server