Re-use existing raw database code for the scanner, without splitting the state yet

[teor2345]

Motivation

For a new database, we can re-use the existing state database code, and pass it a different state path and column family names.

We can use the existing binary serialization formats. Database lookups are fast. Concurrent writing just works.

Rough Design

Here's a rough sketch of the minimum necessary code changes:

• Add db_path, db_version, and column_family_list arguments to ZebraDb::new()
• For example, db_path is state for zebra-state, and could be private-scan for zebra-scan. The other arguments are existing constants.
• Export ZebraDb from zebra-state

Rejected Alternatives

If we re-used the existing state database, we could add a new column family name, and we don't need to change the state path. But the results are harder to delete, because they are mixed with publicly available cached data. This is a security issue.

Related Work

Originally posted by @teor2345 in #7975 (reply in thread)

[mpguerra]

@teor2345 can you please add the estimate for this issue?