IOBuf::getIov

IOBuf::getIov

Summary:
Generate an `fbvector` of `struct iovec` suitable for using with `writev` or
`sendmsg`.

This code is pretty straightforward, but Adam pointed out that something along
these lines has already been done in thrift, so I followed that code closely.
http://fburl.com/11586814

Test Plan:
fbmake runtests

I am using also this in a prototype and it's working there.

Reviewed By: agartrell@fb.com

FB internal diff: D744055