AbstractWorkerThread: add missing synchronization

When running the worker, a call to initWorker() is done to create the
GearmanNIOJobServerConnectionImpl which is synchronized. The server
connection is then added to the worker but that was not synchronized
causing spotbugs to complain:

Inconsistent synchronization of hudson.plugins.gearman.AbstractWorkerThread.conn;
locked 50% of time

src/main/java/hudson/plugins/gearman/AbstractWorkerThread.java

@@ -160,7 +160,9 @@ public void run() {
         while (running) {
             try {
                 logger.info("---- Starting Worker "+ getName() +" ("+new Date().toString()+")");
-                worker.addServer(conn);
+                synchronized(this) {
+                    worker.addServer(conn);
+                }
                 worker.setWorkerID(name);
                 worker.setJobUniqueIdRequired(true);
                 registerJobs();