rtree: improve performance, tuple killing

This patch makes some improvements to the rtree index implementation:

(1) Keep a pin on the scan's current buffer and mark buffer. This avoids
the need to do a ReadBuffer() for each tuple produced by the scan.

(2) Convert a ReleaseBuffer() ; ReadBuffer() pair into
ReleaseAndReadBuffer(). Surely not a huge win, but it saves a lock
acquire/release...

(3) Remove a bunch of duplicated code in rtget.c; make rtnext() handle
both the "initial result" and "subsequent result" cases.

(4) Add support for index tuple killing

(5) Remove rtscancache(): it is dead code, for the same reason that
gistscancache() is dead code (an index scan ought not be invoked with
NoMovementScanDirection).

The end result is about a 10% improvement in index scan performance,
according to contrib/rtree_gist/bench.

These changes (with the exception of #2) are analogous to changes I've
already made for GiST (it's clear that GiST was started as a fork of
rtree). I'm not hugely interested in further improvements to rtree; I
just did this stuff because it is low-hanging fruit and I've already
made the same changes for GiST.

-Neil