Add LCB_ESOCKSHUTDOWN error code for EOF/graceful close

Change-Id: If7183974f1685395d4a35b9f935ed0717b305970
Reviewed-on: http://review.couchbase.org/37340
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>