Use stack allocated StringInfoDatas, where possible

Use stack allocated StringInfoDatas, where possible

Various places that were using StringInfo but didn't need that
StringInfo to exist beyond the scope of the function were using
makeStringInfo(), which allocates both a StringInfoData and the buffer it
uses as two separate allocations.  It's more efficient for these cases to
use a StringInfoData on the stack and initialize it with initStringInfo(),
which only allocates the string buffer.  This also simplifies the cleanup,
in a few cases.

Author: Mats Kindahl <mats.kindahl@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/4379aac8-26f1-42f2-a356-ff0e886228d3@gmail.com