Improve allocation speed and reduce memory copying to increase HTTP pars...

...er performance by 1-2 (and occasionally more) percent in many cases such as benchmark/http/end-vs-write-end.js based on slab allocator todo by Ben.
It's better to call `socket.setEncoding('utf8')` than to repeatedly coerce buffers to strings.  Core has optimized code paths for reading string data from sockets.