Use `itoa` when formatting content-length

[fasterthanlime]

And perhaps generate a magic string to format small lengths without any allocation at all, like we do for status codes?

[fasterthanlime]

And perhaps generate a magic string to format small lengths without any allocation at all, like we do for status codes?

This perf investigation was done when adding benchmarks.

Usage of itoa / an itoa Buffer pool would also be beneficial for content-length chunks in HTTP/1.1