At present, and at least for the medium term (even with the transition to NSE)
the structure of a piece (and thus commP) will remain identical for every size
of sector.
The offline deal flow would benefit greatly if the `lotus client commP`
interface is able to calculate commP without having access to a fully synced
sync, or without even being online.
This is particularly important for filecoin-discover, as we want to allow
miners to spot-check their purchased HDDs, way before they need to accept
the mainnet deal proposals.
See comment/links in node/impl/client/client.go for details on code flow
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