problem was that ha_partition::records was not implemented, thus
using the default handler::records, which is not correct if the engine
does not support HA_STATS_RECORDS_IS_EXACT.
Solution was to implement ha_partition::records as a wrapper around
the underlying partitions records.

The rows column in explain partitions will now include the total
number of records in the partitioned table.

(recommit after removing out-commented code)