feat(chain): Implement DB migration

[lukechampine]

This adds a MigrateDB function that converts a DB from one version to the next. Accordingly, the per-block and per-state versions have been removed.

I've tested this against walletd v0.8.0, though not exhaustively.

Currently there is no live feedback during migration. We could add logging or some other form of callback. We also discussed calling MigrateDB inside NewDBStore; I'm open to that, but it would require changing the function signature if we want logs as well, so we'd need to update a lot of callsites.

[lukechampine]

Ok, new approach. Every key is suffixed with a version. (Prefix would also work fine, and might be better for sorting...hmm)
When we migrate, we check the version byte on every key to determine whether it needs updating.
The old DB keys don't have a version byte at all, though, so instead we check the key length. This is feasible because all the keys have a fixed length, unlike values which can vary in size. This is also why the version is on the key, not the value: if it was on the value, then migrating the old DB (which lacks version bytes) would be ambiguous: you wouldn't be able to tell if the byte was actually a version byte, or part of an old DB value.

I tested this manually by migrating a consensus.db, loading it, and reading from it. When I migrate it again, it's a no-op. When migration is interrupted and resumed, the already-migrated keys are skipped as intended.

[n8maninger]

Looks like this is currently panicking when trying to migrate a fresh Zen walletd v0.8.0 database: failed to create chain store: panic during database initialization: bucket already exists

[lukechampine]

hmm, steps to reproduce? I don't see that error; here's what I did:

• git checkout v0.8.0 && go install ./cmd/walletd
• walletd -network zen
• Wait for a few thousand blocks, then Ctrl-C
• In a test function, call coreutils.OpenBoltChainDB, then chain.MigrateDB, then chain.NewDBStore

[n8maninger]

I did basically the same, but checked out walletd master with coreutils replaced. Let me double check that I didn’t screw up.