libuv: refactor for read2

Change-Id: Ib04e8d8794c4622389bfc1f2d5cd6bb8548f11aa
Reviewed-on: http://review.couchbase.org/36091
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>