Support checksum options

[juliangruber]

• ReadOptions::verify_checksums for read operations, which, by default, don't verify checksums.
• Options::paranoid_checks when opening a db will do aggressive checksum verification

moved over from Level/levelup#12

[ralphtheninja]

@vweevers Anything that's still interesting to implement? I could tackle if that's the case.

[vweevers]

I have no idea. Personally I've never even seen a corrupted db. But maybe I would have, if verify_checksums was enabled? 🤷‍♂

[ralphtheninja]

@peakji Something that floats your boat?

[peakji]

I have never seen a corrupted db. And in my use case, I guess I'll keep verify_checksums disabled for better read performance.

However, there are some blockchain and coin projects using leveldown, maybe checksums are useful there?

[peakji]

BTW, leveldb got some interesting updates recently, we could import after their next release.

[vweevers]

Just discovered that in RocksDB both verify_checksums and paranoid_checks are true by default and in rocksdb we currently only override paranoid_checks to be false. Should we set both to false?

[vweevers]

Can reopen if there's interest.