Integrate with ipfs-blockchain-watcher so that we can readily prove transformed contract data

[i-norden]

Currently, eth-contract-watcher can use the minimal set of standard eth JSON-RPC endpoints exposed by an instance of the ipfs-blockchain-watcher to function in the same exact way as it functions with a geth node. This still requires and anchors data to headers synced by eth-header-sync.

Optionally (and preferably) we should anchor all of the transformed eth-contract-data directly to the eth.headers_cids from ipfs-blockchain-watcher (which themselves are fk linked to the rest of the ethereum data on ipfs). This is the final step to foreign key linking all of the contract data processed by eth-contract-watcher to readily provable data structures.