Avoid invoking PQfnumber in loop constructs

Avoid invoking PQfnumber in loop constructs

When looping over the resultset from a SQL query, extracting the field
number before the loop body to avoid repeated calls to PQfnumber is an
established pattern.  On very wide tables this can have a performance
impact, but it wont be noticeable in the common case. This fixes a few
queries which were extracting the field number in the loop body.

Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Reviewed-by: Nathan Bossart <bossartn@amazon.com>
Discussion: https://postgr.es/m/OS0PR01MB57164C392783F29F6D0ECA0B94139@OS0PR01MB5716.jpnprd01.prod.outlook.com