http: improve chunked res.write(buf) performance

Avoid a costly buffer-to-string operation. Instead, allocate a new
buffer, copy the chunk header and data into it and send that.

The speed difference is negligible on small payloads but it really
shines with larger (10+ kB) chunks. benchmark/http/end-vs-write-end
with 64 kB chunks gives 45-50% higher throughput. With 1 MB chunks,
the difference is a staggering 590%.

Of course, YMMV will vary with real workloads and networks but this
commit should have a positive impact on CPU and memory consumption.

Big kudos to Wyatt Preul (@wpreul) for reporting the issue and providing
the initial patch.

Fixes #5941 and #5944.

Suggested reviewers: @isaacs and @trevnorris