Complete the swap service (megaissue)

[redpig]

The current swap service is very crude:

• collects a wallet of tokens one time from the mint, and
• swaps them out value-for-value
  Once depleted it fails.

The swap service should:

• token management must be safer and have controls
  • handle its cache of tokens automatically
  • the cache/relay should enforce safety policies (rate of withdrawals/etc) rather than minting tokens on the fly (mints should be air-gapped)
• swaps from multiple tokens for one token should be possible
  • to unify change back to denominational values (0.10 + 0.79 + 0.11 = 1.00 or 5 + 3 + 12 = 20, etc)
• swap policy criteria support
  • e.g., only allow swaps of expired tokens or those near the transaction limit
• add support for NAC-based proof of sameness
  • enable a wallet to prove it is the same wallet based on ticket request sameness
  • to swap a token to a new ticket when the prior ticket has expired