@Override
            public void gotData(String key, long cas) {
              rv.setCas(cas);
            }
Asynchronous CAS operation.

@param <T>
@param key the key
@param casId the CAS identifier (from a gets operation)
@param exp the expiration of this object
@param value the new value
@param tc the transcoder to serialize and unserialize the value
@return a future that will indicate the status of the CAS
@throws IllegalStateException in the rare circumstance where queue is too
          full to accept any more requests
src/main/java/net/spy/memcached/MemcachedClient.java