GUAC-1132: Set of active connections should be synchronized.

extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-base/src/main/java/org/glyptodon/guacamole/auth/jdbc/socket/ActiveConnectionMultimap.java

@@ -63,7 +63,7 @@ public class ActiveConnectionMultimap {
             // Get set of active connection records, creating if necessary
             Set<ConnectionRecord> connections = records.get(identifier);
             if (connections == null) {
-                connections = Collections.newSetFromMap(new LinkedHashMap<ConnectionRecord, Boolean>());
+                connections = Collections.synchronizedSet(Collections.newSetFromMap(new LinkedHashMap<ConnectionRecord, Boolean>()));
                 records.put(identifier, connections);
             }