retrychk.c: ADD is not a 'GET' operate, but it is 'SAFE'

Retrying an ADD will lead to errors if the first attempt was a success,
but it will not lead to unintended data manipulation.

Change-Id: Ib32225e22a7ef22338cd3cf3382b02ff97ecf69f
Reviewed-on: http://review.couchbase.org/37597
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>