basic_file_body buffer size and ::sendfile optimization

Hi,

I use Beast in an application which is to be mainly run on SoCs such as the Raspberry Pi.

I have noticed that `boost::beast::file_body` suffers from VERY bad performance when sending large files. When I checked the code (Boost 1.66.0) I was surprised to see a very small buffer (4096 bytes) being used for reading the file, without the possibility of enlarging it easily. That's not nice at all.

I googled and googled and noticed that sendfile support was mentioned a few times, but I didn't find it in either Boost 1.66.0 or the develop version of Beast.

Is it hidden somewhere or what happened to it?