HTTP: Optimise response rendering and response parsing (#21046)

* +htp,ben HttpBlueprint benchmark

* +act bring OptionVal from artery-dev
+htc raw benchmarks

* =htc request parsing benchmark

* +htc,ben add benchmark for longer (raw) json response

* =htc optimise renderHeaders, less Option allocs

* -htc remove FastClock, not quite worth it

akka-actor/src/main/scala/akka/util/ByteString.scala

@@ -382,7 +382,7 @@ object ByteString {
       } else throw new IndexOutOfBoundsException(idx.toString)
     }
 
-    // Avoid `iterator` in performance sensitive code, call ops directly on ByteString instead
+    /** Avoid `iterator` in performance sensitive code, call ops directly on ByteString instead */
     override def iterator: ByteIterator.MultiByteArrayIterator =
       ByteIterator.MultiByteArrayIterator(bytestrings.toStream map { _.iterator })