Sergei's commit ac6b3c44 implemented handler status counters
compensation for underlying handlers like ha_partition.
`index_read_idx_map` is missing there, but it should have been fixed as
well (proof: ha_partition::index_read_idx_map never calls
ha_partition::index_read_map).

Note: all this compensation logic could be broken for subpartitions! (We
can experience double decrement)