refactor subgraph client to use new subgraph implementation in rain.orderbook repo

[mattyg]

The subgraph at https://github.com/rainlanguage/orderbook-subgraph/ is being deprecated and will be replaced by a new implementation in this repo under /subgraph

When that is complete the subgraph schema will need to be updated and the client refactored.

Then we can completed #102

[hardyjosh]

Fixed by #697