Further optimize GIN multi-key searches.

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: 626a120656a75bf4fe64b1d0d83c23cb38d3771a
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2014-01-29T19:24:38Z
Releases: 9.4.0
Further optimize GIN multi-key searches.

When skipping over some items in a posting tree, re-find the new location
by descending the tree from root, rather than walking the right links.
This can save a lot of I/O.

Heavily modified from Alexander Korotkov's fast scan patch.

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