Modify findTransitiveTransfer to call the pathfinder service

[llunaCreixent]

Closes #164

Modify the findTransitiveTransfer method to make request to the pathfinder service API instead of the circles-api.

The core constructor requires two new parameters: pathfinderServiceEndpoint and pathfinderType.

The format of the response of the pathfinder server has changed wrt the cli version. The pathfinder server output is:

{
  "jsonrpc": "2.0",
  "id": "875634786",
  "result": {
    "maxFlowValue": "2000000000000000000000",
    "final": true,
    "transferSteps": [
      {
        "from": "0x0001",
        "to": "0x0002",
        "token_owner": "0x0003",
        "value": "158277265717615256803"
      },
      {
        "from": "0x0001",
        "to": "0x0002",
        "token_owner": "0x0004",
        "value": "158277265717615256803"
      }
    ]
  }
}

While the cli output is:

{
  "status": "ok",
  "data": {
    "from": "0x0001",
    "to": "0x0002",
    "maxFlowValue": "2000000000000000000000",
    "processDuration": 74697,
    "transferValue": "2000000000000000000000",
    "transferSteps": [
      {
        "from": "0x0001",
        "to": "0x0002",
        "value": "158277265717615256803",
        "step": 0,
        "tokenOwnerAddress": "0x0003"
      },
      {
        "from": "0x0001",
        "to": "0x0002",
        "value": "158277265717615256803",
        "step": 1,
        "tokenOwnerAddress": "0x0004"
      }
    ]
  }
}

The optional parameters for the pathfinder compute_transfer method have changed: hops is not available anymore. max_transfers is used by the pathfinder service to try to remove individual transfers to stay below the limit, even if this results in less value being transferred.

Furthermore, the logic of token.transfer is improved by trying first a direct transfer with the token of the 'to' address, and if not possible then try with the token of the 'from' address.

Moreover, we adapted the tests to work with automatic block mining enabled (blockTime option in ganache): adding loops to wait for the data to be available in the blockchain.

The Token tests are parametrized to either deploy a core instance with the pathfinderType as cli or server.

While implementing this PR, I've found out something interesting: the checkSendLimit method of the core checks the sent limit of the token of the 'from' address, which is contradictory with the policy of "returning tokens to their owners". This method is only used by myxo when calculating maxFlow. I suggest we refactor the method to include the tokenOwner as param too. #168

Another discussion is whether the max_steps = 52 is too large. #166

Finally there is a possible issue to explore: if we transfer all the balance of a token that was supposed to be used for paying the relayer for the tx gas, would it generate an error? #167

[juanenrisley]

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