Replace pq_getbytes(&ch, 1) calls with pq_getbyte(), which is easier

Replace pq_getbytes(&ch, 1) calls with pq_getbyte(), which is easier
to use and significantly faster.  This tweak saves 25% (!) of the runtime
of COPY IN in a test with 8000-character lines.  I wouldn't normally
commit a performance improvement this late in the cycle, but 25% got
my attention...