Better use of AtomicBoolean (#1210)

Author: scottf

src/main/java/io/nats/client/impl/NatsConnectionWriter.java

@@ -200,8 +200,7 @@ public void run() {
                     sendMessageBatch(msg, dataPort, stats);
                 }
 
-                if (flushBuffer.get()) {
-                    flushBuffer.set(false);
+                if (flushBuffer.getAndSet(false)) {
                     dataPort.flush();
                 }
             }