Use one stringbuffer for all rows printed in printtup.c.
This avoids newly allocating, and then possibly growing, the
stringbuffer for every row. For wide rows this can substantially
reduce memory allocator overhead, at the price of not immediately
reducing memory usage after outputting an especially wide row.
Author: Andres Freund
Discussion: https://postgr.es/m/
20170914063418.sckdzgjfrsbekae4@alap3.anarazel.de