patch for new feature: Buffer Cache Hibernation

Hi,

I am working on new feature `Buffer Cache Hibernation' which enables
postgres to keep higher cache hit ratio even just started.

Postgres usually starts with ZERO buffer cache. By saving the buffer
cache data structure into hibernation files just before shutdown, and
loading them at startup, postgres can start operations with the saved
buffer cache as the same condition as just before the last shutdown.

Here is the patch for 9.0.3 (also tested on 8.4.7)
http://people.freebsd.org/~iwasaki/postgres/buffer-cache-hibernation-postgresql-9.0.3.patch

The patch includes the following.
- At shutdown, buffer cache data structure (such as BufferDescriptors,
BufferBlocks and StrategyControl) is saved into hibernation files.
- At startup, buffer cache data structure is loaded from hibernation
files and buffer lookup hashtable is setup based on buffer descriptors.
- Above functions are enabled by specifying `enable_buffer_cache_hibernation=on'
in postgresql.conf.

Any comments are welcome and I would very much appreciate merging the
patch in source tree.

Have fun and thanks!