Add /version API

[garyyu]

Now we have these wallet APIs in 1.1.0 version:

• /v2/foreign/**
• /v2/owner/**
• /v1/wallet/foreign/**
• /v1/wallet/owner/**

How about add a /version API since 1.1.0 version?

1. We can use this /version API to make 1.1.0 version wallet sending to be compatible with 1.0.x version receiving. The 1.0.x version will return 404 Not Found for this API.
2. Sending side can detect the receiver's version in all the future versions since 1.1.0.

[yeastplume]

This is already covered here: #87

I don't think we want to change the endpoint each time the api changes. V1 is the http api, V2 is the json-rpc api, but there is another version number within these that can be retrieved by the check_version function.

[garyyu]

Thanks.
Close this one since check_version works for that.

$ curl -0 -XPOST http://127.0.0.1:3415/v2/foreign --data '{"jsonrpc":"2.0","method":"check_version","id":1,"params":[]}'


{
  "id": 1,
  "jsonrpc": "2.0",
  "result": {
    "Ok": {
      "default_slate_version": 2,
      "foreign_api_version": 2
    }
  }
}