couchbase Cfg on CAS of 0 should do Add not Set

Another concurrency issue, found by Nimish! @nimishzynga

If two concurrent cbgt's are initially racing to read an empty Cfg, then they'll both get a not-found response and also CAS of 0.

Then later, when they each try to do a Cfg.Set(), the couchbase Cfg implementation currently (incorrectly) does a bucket.Set() when the CAS is 0. Instead, the implementation should do a bucket.Add() when the CAS is 0 in order to have a clean concurrency race winner.