domain: use snapshot interface to get commit ts of schema version

[cfzjywxk]

Enhancement

Currently, it's needed to get commit_ts of a schema version when inserting schema cache, the mvccGetByKey interface is used to do so. But this makes the entire schema reload process appear inconsistent. Snapshot-related interfaces are used during the reading of schema version and schema diff version, while mvcc interfaces are used when it's necessary to read the commit_ts of a schema diff transaction. Moreover, during the process of using mvccGetByKey, we have already encountered the following issues leading to schema cache invalidation:

• unexpected error: There is no Write MVCC info for the schema version #47807, the region error is not handled using mvcc interfaces
• domain: lock error is not handled getting commit ts of schema diff key #48281, the lock result is not handled using mvcc interfaces