FM-137: Ethereum API filter methods

[aakoshh]

Part of consensus-shipyard/ipc#342
Closes consensus-shipyard/ipc#328

Adds API methods related to filters:

• eth_newFilter
• eth_newBlockFilter
• eth_pendingTransactionFilter
• eth_uninstallFilter
• eth_getFilterChanges

Caveats:

• Instead of pending transactions, we return transactions which are included in blocks, as that's what we are able to subscribe to
• Logs returned in subscriptions don't have block_hash, transaction_index and log_start_index fields as these would require further queries to the API, with caching to avoid hammering the node.

TODO:

• Check how Etherscan deals with the extra fields in logs.
• Follow up with a test to parse the logs with the ethers machinery.