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.
`Date.now()` or, even better, `process.hrtime()`.  Although normally it's the total running time of the benchmark that matters, you don't have to time each request.

There's actually a case to be made for not doing that inside the benchmark.  Obtaining the current time can be quite expensive on virtualized systems because the operation has to emulated by the host operating system.  It's easily 100x or 1000x times more expensive than on real hardware and can skew the numbers heavily.