Scan consistency guarantee

For now, although we already pass mvcc back to client, and client will send this mvcc to RS when reopening a scanner, we still can not make sure that the region/row level consistency is safe as compaction happens after region move but before the scanner reopen may delete some cells which mvcc are before the mvcc given by client.

This issue aims to try our best to guarantee region level consistency, and if not, we need to tell users we can not make it.