def replace(key, value, ttl=nil, options=nil)
      perform(:replace, key, value, ttl_or_default(ttl), 0, options)
    end
def replace(key, value, ttl=nil, options=nil)
      perform(:replace, key, value, ttl_or_default(ttl), 0, options)
    end
Conditionally add a key/value pair, only if the key already exists
 on the server.  Returns truthy if the operation succeeded.