[docdb] Move tablet schema information into rocksdb

[bmatican]

Jira Link: DB-2149
Currently, we store some type of metadata (eg: raft, schema) on disk in a separate file, in PB format. This makes it hard to version correctly and reason about atomicity guarantees.

One option we've discussed before is moving some of this into rocksdb, say, as a set of custom keys. This would ensure that it's versioned, due to our mvcc setup.

On the data integrity piece, it might help with issues such as #5271

On the data versioning piece, it could help with issues such as #7135, #7495