server_negotiate: add "sasl" to sock->subsys

Change-Id: I294da0e963e8cf1729b0164579f42877e63df2b1
Reviewed-on: http://review.couchbase.org/36084
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>