Improve the performance of nested loop join in the case of partitioned inner table

Hi, hackers

I propose to slightly improve the performance of nested loop join in the
case of partitioned inner table.
As I see in the code, the backend looks for the partition of the inner
table each time after fetch a new row from the outer table.
These searches can take a significant amount of time.
But we can skip this step if the nested loop parameter(s) was(re) not
changed since the previous row fetched from the outer table

The draft patch is attached.