read is reused for successive attributes, instead of being deleted and
recreated from scratch for each value read in.  This reduces palloc/pfree
overhead a lot.  COPY IN still seems to be noticeably slower than it was
in 6.5 --- we need to figure out why.  This change takes care of the only
major performance loss I can see in copy.c itself, so the performance
problem is at a lower level somewhere.