Add read-only stats() to ReadTransaction for concurrent access

[mvdnbrk]

Description:
In Redb v2.4.0, stats() is only available on WriteTransaction, requiring an exclusive lock via begin_write(). This prevents concurrent read-only operations when another process (e.g., a server) holds the database, causing errors like Database already open. Cannot acquire lock. in downstream projects like ord.

Use Case:
In ord v0.22.2 (src/index.rs:568), info() uses:

let stats = self.database.begin_write()?.stats()?;
let rtx = self.database.begin_read()?;

when the ord server runs (e.g., ./ord server), running ord index info fails with:

error: failed to open index: Database already open. Cannot acquire lock.

This happens because begin_write() conflicts with the server’s lock, even though stats() is a read-only operation.

Adding stats() method to ReadTransaction would allow read-only access to database stats without requiring an exclusive lock.

[cberner]

stats() access state that is only available in write transaction -- the page allocation stats -- so unfortunately this can't be added to read-only transactions