As we are making the `DatabaseName::db()` private, all the call sites of it should be changed to replace it with a proper function.

• For serialization nss/db cases: Before we start to support shard in serverless, there is no tenant in shard. So, we decided to call `DatabaseNameUtil::serialize` function to serialize the DatabaseName object to string. 
• For check empty db, call `.isEmpty()`
• For create opmsgrequest, call `create` with DatabaseName object.