Increase maximum number of clog buffers.

Increase maximum number of clog buffers.

Benchmarking has shown that the current number of clog buffers limits
scalability. We've previously increased the number in 33aaa139, but
that's not sufficient with a large number of clients.

We've benchmarked the cost of increasing the limit by benchmarking worst
case scenarios; testing showed that 128 buffers don't cause a
regression, even in contrived scenarios, whereas 256 does

There are a number of more complex patches flying around to address
various clog scalability problems, but this is simple enough that we can
get it into 9.6; and is beneficial even after those patches have been
applied.

It is a bit unsatisfactory to increase this in small steps every few
releases, but a better solution seems to require a rewrite of slru.c;
not something done quickly.

Author: Amit Kapila and Andres Freund
Discussion: CAA4eK1+-=18HOrdqtLXqOMwZDbC_15WTyHiFruz7BvVArZPaAw@mail.gmail.com