V2 Slate Compatibility

[yeastplume]

This restores wallet-to-wallet compatibility between V3.0.0 and V2.0.0 by re-introducing the V2 slate. The rules for conversion are:

When sending via HTTP (including TOR), the slate version on the remote side is checked and:

• If V3 support is reported back, send slate as a V3 slate
• If V2 only is reported back:
  • Downgrade slate to V2 before sending if payment_proof is None
  • If payment_proof is Some then stop the transaction with an error saying proofs aren't supported by the other side
  • If ttl_cutoff_height is some, warn the user that it will be ignored by the other side and dropped by them in the slate, but continue to downgrade to V2

When outputting a file (either via send or invoice):

• If both payment_proof and ttl_cutoff_height are None, downgrade slate to V2 before outputting
• If either payment_proof or ttl_cutoff_height are Some, output V3 slate with a warning that the other side must be upgraded to grin-wallet v3.0.0 or later

Tested sending back and forth between v2.0.0 and 3.0.0 wallets, sends via direct http and files, also invoices.

[quentinlesceller]

Currently reviewing but just to let you know that the failing tests is because of:

thread 'main' panicked at '(left != right) 
left: {
  "id": 1,
  "jsonrpc": "2.0",
  "result": {
    "Ok": {
      "foreign_api_version": 2,
      "supported_slate_versions": [
        "V3",
        "V2"
      ]
    }
  }
}
right: {
  "id": 1,
  "jsonrpc": "2.0",
  "result": {
    "Ok": {
      "foreign_api_version": 2,
      "supported_slate_versions": [
        "V3"
      ]
    }
  }
}'

Defined here https://github.com/mimblewimble/grin-wallet/blob/master/api/src/foreign_rpc.rs#L56
So you don't have to go on Azure website :).

[quentinlesceller]

👍 Tested between V2 and V3 node using http and file.