hook to enable body to be streamed

[dom96]

If httpbeast receives large body it will read that all into memory. Ideally we should offer a way to stream this.

Ideal solution here is to utilise sendfile

[PhilippMDoerner]

If there is any sort of feedback I can provide with my prologue project (nimstoryfont) which deals exactly with uploading large files I will happily do so. I will not yet offer any sort of direct contribution, mostly because I don't have that much of an idea in dealing truly low level stuff, but am happy to test stuff outside of that.