Microbench backport issue

Motivation:
The microbench code in 4.0 lives in src/test while in 4.1 and master it lives in src/main. A backport of a patch did not account for this.

Modifications:
- Move the benchmark to the src/test directory
- Update new benchmark package info

Result:
4.0 branch can now build again.

microbench/src/test/java/io/netty/microbench/common/AppendableCharSequenceBenchmark.java

@@ -12,7 +12,7 @@
  * or implied. See the License for the specific language governing permissions and limitations under
  * the License.
  */
-package io.netty.microbenchmark.common;
+package io.netty.microbench.common;
 
 import java.util.Random;