Support `logs` w/ `eth_subscribe` RPC

[fubuloubu]

Didn't see another issue for this, but am interested to see it implemented

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With #547 implemented, what would it take for helios to support logs subscription type?

[ncitron]

This is not terribly hard to support, and is on the roadmap. We'll certainly be implementing it. We already support eth_getLogs and there's some work being done to support eth_getFilterChanges, both of which should provide most the scaffolding for supporting log subscriptions.