Implement toString for non-blocking semaphores (AsyncHttpClient#1376)

Author: stepancheg

client/src/main/java/org/asynchttpclient/netty/channel/NonBlockingSemaphore.java

@@ -45,4 +45,10 @@ public boolean tryAcquire() {
             }
         }
     }
+
+    @Override
+    public String toString() {
+        // mimic toString of Semaphore class
+        return super.toString() + "[Permits = " + permits + "]";
+    }
 }

client/src/main/java/org/asynchttpclient/netty/channel/NonBlockingSemaphoreInfinite.java

@@ -31,4 +31,9 @@ public void release() {
     public boolean tryAcquire() {
         return true;
     }
+
+    @Override
+    public String toString() {
+        return NonBlockingSemaphore.class.getName();
+    }
 }