2Q implementaion for PostgreSQL buffer replacement.

Hi.

I implement 2Q algorithm to PostgreSQL for buffer management , instead
of LRU.
It's known as low overhead and high performance than LRU. If you have
some interests , see following URL.

http://www.vldb.org/conf/1994/P439.PDF

In my test (pgbench -S) , it improves 4% cache hit rate and 2% up
performance comparing from LRU.

Do you have any interest about this patch?

--
Yutaka tanida <yutaka@nonsensecorner.com>
http://www.nonsensecorner.com/