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.
