For bulk write operations (eg COPY IN), use a ring buffer of 16MB instead

For bulk write operations (eg COPY IN), use a ring buffer of 16MB instead
of the 256KB limit originally enforced by a patch committed 2008-11-06.
Per recent test results, the smaller size resulted in an undesirable decrease
in bulk data loading speed, due to COPY processing frequently getting blocked
for WAL flushing.  This area might need more tweaking later, but this setting
seems to be good enough for 8.4.