MDEV-17297: stats.records=0 for a table of Archive engine when it has rows, when we run ANALYZE command

Archive storage engine assumed that any query that attempts to read from
the table will call ha_archive::info() beforehand. ha_archive would flush
un-written data in that call (this would make it visible for the reads).

Break this assumption. Flush the data when the table is opened for reading.

This way, one can do multiple write statements without causing a flush, but
as soon as we might need the data, we flush it.