ByteString optimisations of methods in HTTP parsing hot-path (#20994)

* =act #20992 prepare benchmarks for ByteString optimisations

* =act #20992 optimise common ByteString operations: drop,take,slice...

* =act,htc #15965 add ByteString.decodeString(java.nio.charsets.Charset)

project/MiMa.scala

@@ -904,6 +904,10 @@ object MiMa extends AutoPlugin {
 
         // #20543 GraphStage subtypes should not be private to akka
         ProblemFilters.exclude[DirectAbstractMethodProblem]("akka.stream.ActorMaterializer.actorOf")
+      ),
+      "2.4.9" -> Seq(
+        // #20994 adding new decode method, since we're on JDK7+ now
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.util.ByteString.decodeString")
       )
     )
   }