Http perf2

This pull-req optimize the Http codec to be much faster:
* Not parse the whole header again and again just because something is missing
* Eliminate parsing the same line multi times 
* Reduce object creation
* Allow to disable validation of decoded headers
* Not validate known header names 
...

@trustin please review. I may also rewrite the chunkSize parsing and trailing header parsing at some point but I don't think it is "too important" . Once reviewed I will squash it, but I thought it may be better to review it as it is for now