Update Nimbus codebase to use the new nim-rocksdb API.

[web3-developer]

The new version of nim-rocksdb makes the following breaking changes:

• RocksDbInstance now named RocksDbReadWriteRef or RocksDbReadOnlyRef depending on readOnly mode.
• contains renamed to keyExists.
• del renamed to delete and no longer returns flag indicating if a record was deleted.
• Backup features moved to BackupEngineRef type.
• Low level c api moved to rocksdb/lib/librocksdb and is not exported by default.
• C API types are updated to be objects so ptr prefix is required. This was required in order to update the header file wrapper to the latest version.
• The RocksDbRef types no longer expose internal fields.

Due to these above changes a lot of files needed to be updated.

[web3-developer]

While looking through the code I also discovered a memory leak due to calling allocCStringArray without a free. See docs for that function here: https://nim-lang.org/docs/system.html#allocCStringArray%2CopenArray%5Bstring%5D

I've pushed a fix so that we don't alloc a new cstring array in these cases.

[web3-developer]

This is the fix for the memory leak. Similar change in two other places.

[jangko]

Whenever possible, where there is no conflict between symbols, perhaps we can use valueOr, isOkOr, etc to improve code readability. I know there is still a lot of old code using isErr, well, they are old code.

example:

let backupRes = openBackupEngine(backupsDir).valueOr:
     return err(error)

# or
let backupRes = ? openBackupEngine(backupsDir)  # if both callee and caller have identical return type

[web3-developer]

Thanks, yes looks much cleaner. I'll update.