Add read2 API

This adds an alternate read API for completion-style I/O. It uses
explicit buffer passing rather than implicitly relying on the deprecated
'readbuf' and 'bufinfo' structures within the sockdata_t

Change-Id: I3a17136b9a45b1581250a897e424e319bffbeaa6
Reviewed-on: http://review.couchbase.org/35869
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>