def set_cas(key, value, cas, ttl=nil, options=nil)
      ttl ||= @options[:expires_in].to_i
      perform(:set, key, value, ttl, cas, options)
    end
def set_cas(key, value, cas, ttl=nil, options=nil)
      ttl ||= @options[:expires_in].to_i
      perform(:set, key, value, ttl, cas, options)
    end
Set the key-value pair, verifying existing CAS.
 Returns the resulting CAS value if succeeded, and falsy otherwise.