sdk/state, sdk/agent: add memos to payments

[leighmcculloch]

Payments should have memos so they can be identifiable on the terms of the participants. Participants may communicate about a payment ahead of sending a payment and the memo will provide a way to annotate or identify the payment in whatever way they require.

Note that this is similar but different to #339. #339 is concerned with uniquely identifying payments with an identifier that the state machine attaches to close agreements. That would not be useful in the situation described above where an identifier must be assigned prior to the payment being made.

This is useful for things like buffered channels, something I'm experimenting with right now for #340.

Refs:

• sdk/state: give close agreements identifiers separate to their iteration number #339
• sdk, examples/benchmark: experiment with a buffered channel #340