Replace polling with subscriptions where possible (in @iov/ethereum)

[ethanfrey]

Once we fully understand the subscription system and the logs/topics, we should use that to:

1. listen for block headers (basically done)
2. listen for transactions that change certain state
3. use those transaction subscriptions to trigger a query to implement watch like functionality without polling.

If there are important items that are impossible to subscribe to, make another issue delimiting them and consider if there is a scrapper we could use to provide that info, eg. vulcanize