Migrate to latest `darkside.proto` to help fix Darkside tests

[str4d]

Depends on https://github.com/zcash/lightwalletd/pull/448.

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[str4d]

Rebased on current main. I think this PR can be reviewed and merged (as the corresponding lightwalletd PR has been), and then we can continue fixing the Darkside tests in subsequent PRs.

[LukasKorba]

I pulled lightwalletd, used make to build the latest server and ran all dark side test with this result:

I don't know if this is expected or not, I remember str4d to talk about fixes of fundamental issues but still, after thiese fixes the tests need to be iterated to pass...

[LukasKorba]

Approving as I also think the purpose of this PR is fulfilled.