Support index condition pushdown within partitioned tables.
- ha_partition will pass the pushed index condition into all of the used
  partitions.
  - We require that all of the partitions to handle the pushed index
    condition in the same way.
- When using ICP, one may read rows (e.g. call h->index_read_map(buf, ...)
  only to buf= table->record[0], for two reasons:
  * Pushed index condition's Item_field objects point into record[0]
  * InnoDB requires this: it calls offset() which assumes record[0].
  So, when using ICP, ha_partition will read partition records to
  table->record[0] and then will copy record away if it needs it to be
  elsewhere.