WIP: SP-GiST, Space-Partitioned GiST

Hi there,

attached is WIP-patch for 9.2 development source tree, which provides
implementation of SP-GiST (prototype
was presented at PGCon-2011, see
http://www.pgcon.org/2011/schedule/events/309.en.html and presentation
for details) as a core feature. Main differences from prototype version:

1. Now it's part of pg core, not contrib module
2. It provides more operations for quadtree and suffix tree
3. It uses clustering algorithm of nodes on disk and has much better
utilization of disk space. Fillfactor is supported
4. Some corner cases were eliminated
5. It provides support for concurency and recovery (inserts are
logged, supports for deletes, and log replay will be added really
soon)

So, now code contains almost all possible overhead of production code
and we ask hackers to test performance on real data sets. We expect
the same performance for random data (since almost no overlaps) and
much better performance on real-life data, plus much better index
creation time. Also, we appreciate your comments and suggestions about
API.

Regards,

Oleg