Including the setup code in the benchmark method to avoid JMH Invocation level hiccups.

Motivation:

The usage of Invocation level for JMH fixture methods (setup/teardown) inccurs in a significant impact in
in the benchmark time (see org.openjdk.jmh.annotations.Level documentation).

When the benchmark and the setup/teardown is too small (less than a milisecond) the Invocation level might saturate the system with
timestamp requests and iteration synchronizations which introduce artificial latency, throughput, and scalability bottlenecks.

In the HeadersBenchmark, all benchmarks take less than 100ns and the Invocation level setup offsets the measurement considerably.
As fixture methods is defined for the entire class, this overhead also impacts every single benchmark in this class, not only
the ones that use the emptyHttpHeaders object (cleaned in the setup).

The recommended fix patch here is to include the setup/teardown code in the benchmark where the object is used.

Modifications:

Include the setup/teardown code in the relevant benchmark methods.
Remove the setup/teardown method of Invocation level from the benchmark class.

Result:

We run all benchmarks from HeadersBenchmark 10 times with default parameter, we observe:
- Benchmarks that were not directly affected by the fix patch, improved execution time.
    For instance, http2Remove with (exampleHeader = THREE) had its median reported as 2x faster than the original version.
- Benchmarks that had the setup code inserted (eg. http2AddAllFastest) did not suffer a significant punch in the execution time,
as the benchmarks are not dominated by the clear().

Environment:
Tests run on a Computational server with CPU: E5-1660-3.3GHZ  (6 cores + HT), 64 GB RAM.

microbench/src/main/java/io/netty/microbench/headers/HeadersBenchmark.java

@@ -102,14 +102,6 @@ public class HeadersBenchmark extends AbstractMicrobenchmark {
         emptyHttp2HeadersNoValidate = new DefaultHttp2Headers(false);
     }
 
-    @Setup(Level.Invocation)
-    public void setupEmptyHeaders() {
-        emptyHttpHeaders.clear();
-        emptyHttp2Headers .clear();
-        emptyHttpHeadersNoValidate.clear();
-        emptyHttp2HeadersNoValidate.clear();
-    }
-
     @Benchmark
     @BenchmarkMode(Mode.AverageTime)
     public void httpRemove(Blackhole bh) {
@@ -183,30 +175,35 @@ public class HeadersBenchmark extends AbstractMicrobenchmark {
     @BenchmarkMode(Mode.AverageTime)
     public void httpAddAllFastest(Blackhole bh) {
         bh.consume(emptyHttpHeadersNoValidate.add(httpHeaders));
+        emptyHttpHeadersNoValidate.clear();
     }
 
     @Benchmark
     @BenchmarkMode(Mode.AverageTime)
     public void httpAddAllFast(Blackhole bh) {
         bh.consume(emptyHttpHeaders.add(httpHeaders));
+        emptyHttpHeaders.clear();
     }
 
     @Benchmark
     @BenchmarkMode(Mode.AverageTime)
     public void http2AddAllFastest(Blackhole bh) {
         bh.consume(emptyHttp2HeadersNoValidate.add(http2Headers));
+        emptyHttp2HeadersNoValidate.clear();
     }
 
     @Benchmark
     @BenchmarkMode(Mode.AverageTime)
     public void http2AddAllFast(Blackhole bh) {
         bh.consume(emptyHttp2Headers.add(http2Headers));
+        emptyHttp2Headers.clear();
     }
 
     @Benchmark
     @BenchmarkMode(Mode.AverageTime)
     public void http2AddAllSlow(Blackhole bh) {
         bh.consume(emptyHttp2Headers.add(slowHttp2Headers));
+        emptyHttp2Headers.clear();
     }
 
     private static final class SlowHeaders implements Headers<CharSequence, CharSequence, SlowHeaders> {