CCBC-297: UV: Handle read_cb with no data and EAGAIN

Change-Id: I278816806fbb285aebd5ad319c3853e515e5803c
Reviewed-on: http://review.couchbase.org/33192
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>