HttpObjectDecoder performance improvements.

This two commits improve performance of the HttpObjectDecoder.
Before I got 400k req/sec with vert.x testsuite, after 425k req/sec.

Basically this changes do:
* Minimize char array copies because of less toString() calls
* Re-use StringBuilder to minimize allocation cost and expand costs
* Try to parse HttpVersion without convert String to uppercase 

This stuff showed up in the profiler...

I would love to merge this in. 

@jpinner @trustin please review