Optimize method io.netty.util.concurrent.DefaultPromise.await0(...) (#11504)

Motivation:

For the code pattern of `Object.wait(...)`  in `io.netty.util.concurrent.DefaultPromise.await0(...)`, it's better to follow the recommended code pattern according to [Object.wait(...)'s doc](https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#wait()):
```
synchronized (obj) {
     while (<condition does not hold>)
     obj.wait();
     ... // Perform action appropriate to condition
}
```

Modification:
Change the `Object.wait(...)`'s code pattern in `io.netty.util.concurrent.DefaultPromise.await0(...)`.

Result:

The `Object.wait(...)`'s code pattern in `io.netty.util.concurrent.DefaultPromise.await0(...)` meets the Java doc.

common/src/main/java/io/netty/util/concurrent/DefaultPromise.java

@@ -666,15 +666,14 @@ public class DefaultPromise<V> extends AbstractFuture<V> implements Promise<V> {
 
         checkDeadLock();
 
-        long startTime = System.nanoTime();
+        // Start counting time from here instead of the first line of this method,
-        long waitTime = timeoutNanos;
+        // to avoid/postpone performance cost of System.nanoTime().
+        final long startTime = System.nanoTime();
+        synchronized (this) {
             boolean interrupted = false;
             try {
-            for (;;) {
+                long waitTime = timeoutNanos;
-                synchronized (this) {
+                while (!isDone() && waitTime > 0) {
-                    if (isDone()) {
-                        return true;
-                    }
                     incWaiters();
                     try {
                         wait(waitTime / 1000000, (int) (waitTime % 1000000));
@@ -687,22 +686,22 @@ public class DefaultPromise<V> extends AbstractFuture<V> implements Promise<V> {
                     } finally {
                         decWaiters();
                     }
-                }
+                    // Check isDone() in advance, try to avoid calculating the elapsed time later.
                     if (isDone()) {
                         return true;
-                } else {
+                    }
+                    // Calculate the elapsed time here instead of in the while condition,
+                    // try to avoid performance cost of System.nanoTime() in the first loop of while.
                     waitTime = timeoutNanos - (System.nanoTime() - startTime);
-                    if (waitTime <= 0) {
+                }
                 return isDone();
-                    }
-                }
-            }
             } finally {
                 if (interrupted) {
                     Thread.currentThread().interrupt();
                 }
             }
         }
+    }
 
     /**
      * Notify all progressive listeners.