feat: implement value_schema_manager::get_value_schema

[levy5307]

What problem does this PR solve?

proposal https://github.com/apache/incubator-pegasus/blob/master/rfcs/2020-10-09-data-version-v3.md

implement value_schema_manager::get_value_schema

Checklist

Tests

• Unit test

[hycdong]

The old value_schema is a table level concept defined in data_version in meta column family. Now, value_schema version 2 updates it, value_schema becomes a value level concept, a table is permitted having different value schema. However, it will raise two problems.
The first problem is the compatibility problem. New server version won't be downgraded because the old server can not figure out value_schema version 2 data. I think it is okay for new server can not downgrade, but our recent releases are almost all not-downgraded, should we merge this commit into master branch?
The second problem is the data_version in meta cf. This value is actually useless for new server, but it can not be removed for version0 and version1, would you have any solution to remove this value in future?

[levy5307]

The old value_schema is a table level concept defined in data_version in meta column family. Now, value_schema version 2 updates it, value_schema becomes a value level concept, a table is permitted having different value schema. However, it will raise two problems.
The first problem is the compatibility problem. New server version won't be downgraded because the old server can not figure out value_schema version 2 data. I think it is okay for new server can not downgrade, but our recent releases are almost all not-downgraded, should we merge this commit into master branch?
The second problem is the data_version in meta cf. This value is actually useless for new server, but it can not be removed for version0 and version1, would you have any solution to remove this value in future?

I don't think so. Because value_schema version 2 will not updates data version in meta column family. It only reads from it.