Adding a memo field to the Tx output

[azuchi]

Allow arbitrary data to be registered for Tx outputs. Until now, metadata has been added by adding OP_RETURN for layer 2 protocols, etc., but it would be useful for these if metadata could be registered for each output itself.

This introduction uses the second feature bits of the Tx, since the format of the Tx changes. When this bit is standing, this new Tx format is applied. A limit of 500 bytes is set for the memo field as policy, but this can be changed by node setting (similar to the policy for the 80bytes limit in OP_RETURN).