lcb_config_transport: add constant indicating maximum value

Change-Id: I5ef2e879986e21d5d77961a45845c7bab3b433fd
Reviewed-on: http://review.couchbase.org/37787
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>