Replace occurences of codeHash, code_hash with checkSum

[DimitrisJim]

Feedback from Simon's peer review:

Consistently rename code_hash -> checksum. Those are two terms for the same thing, which is confusing. cosmwasm-vm/wasmvm uses “checksum” everywhere.

We might want to use a different name for the hex encoded checksums returned by the QueryCodeHashesRequest and the one supplied to QueryCodeRequest. checkSumHex is an unimaginative first stab at it.

Can be done after feature branch is merged.

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[DimitrisJim]

note: will probably require updates to documentation

[colin-axner]

This can be closed?

[DimitrisJim]

This can be closed?

keeping open for doc changes, wanted to push through main changes and keep the docs amendments in a follow up