Simpler payment channels

[markspanbroek]

Replace the Nitro state channel PoC implementation with simpler 1-to-1 payment channels.

Rationale: we need to have a fast execution layer for our smart contracts anyway, might as well use that to open 1-to-1 payment channels, without the complexity of Nitro state channels.

• [ ] Implement simple 1-to-1 state channels in Nim
• [ ] Rework block exchange (including tests) to use simple channels
• [ ] Check that channel is created on-chain before accepting payments
• [ ] After sufficient payments have been received, they should be cached in on-chain

[Bulat-Ziganshin]

Just now in https://github.com/status-im/nim-codex/tests/codex/blockexchange/engine/testblockexc.nim#L161-L173

0. in setup, we exchange blocks between node1 and node2. These blocks are paid and since they have random prices, we start the test with non-zero balance, so our payment test doesn't test much

1. In order to fix that, we should assign equal prices to blocks on node1 and node2. And better they will be small, so we don't run out of money when performing Balance.move(): https://github.com/status-im/nim-nitro/blob/main/nitro/wallet/balances.nim#L33 We can return back to:

   pricing1.price = 1
   pricing2.price = 1

2. We need to add test that balance is 0 without trading:

   test "Balance is 0":
   let channel = !peerCtx1.paymentChannel
   let wallet = nodeCmps2.wallet
   check wallet.balance(channel, Asset) == 0

   So, we need two changes: price=1 and test "Balance is 0". But the later will not work till we will fix that bug around nitro

Extra info: I made pseudo-PR to check balance on unices: https://github.com/status-im/nim-codex/pull/301 is just current branch main with a few extra echo statements (and price:=0)

And these echo statements https://github.com/status-im/nim-codex/blob/7b62e82b3540c2de06f30a0eaacc9ba6aff785ba/tests/codex/blockexchange/engine/testblockexc.nim#L161-L173 print the following:

pricing1 some(0)
pricing2 some(0)
balance0 de0b6b3a7640000

balance after de0b6b3a7640000
pricing1 some(0)
pricing2 some(0)

So, on unices we get an inheritance (nonzero "balance0").

It looks like a bug, isn't it? Moreover, it seems that we always had this bug, but #248 fixed it on windows. I bet that this is an error somewhere deep in libraries.