=core Make use of statefulMap instead of statefulMapConcat.

[He-Pin]

Motivation:
Change to use statefulMap instead when not concating.

[He-Pin]

I ran a benchmark locally just now @johanandren :

[info] Do not assume the numbers tell you what you want them to tell.
[info] Benchmark                                                Mode  Cnt         Score        Error  Units
[info] MessageToFrameRendererBenchmark.benchNewStreamedFrames  thrpt   10  15148931.446 锟斤拷  99406.646  ops/s
[info] MessageToFrameRendererBenchmark.benchOldStreamedFrames  thrpt   10  13296638.654 锟斤拷 553890.759  ops/s

with

Jmh/run -i 10 -wi 10 -f1 -t1 .MessageToFrameRendererBenchmark.

And I think this is OK.