Use `sendfile()` in PollMessages for TCP

[hubcio]

This is supported only on Linux.

[iamazy]

Hi, @hubcio I'm unfamiliar with the usage of sendfile and would like to inquire how to read bytes from a file into Vec using sendfile

[hubcio]

@iamazy unfortunately, this API is blocking so it'll be pain in the ass to do it in tokio. we'd have to implement separate thread pool or use tokio::block_on