Small GIN optimizations (after 9.4)

While hacking on the GIN patches, I've come up with a few different
ideas for improving performance. It's too late for 9.4, but I'll list
them here if someone wants to work on them later:

* Represent ItemPointers as uint64's, to speed up comparisons.
ginCompareItemPointers is inlined into only a few instructions, but it's
still more expensive than a single-instruction 64-bit comparison.
ginCompareItemPointers is called very heavily in a GIN scan, so even a
small improvement there would make for a noticeable speedup. It might be
an improvement in code clarity, too.

* Keep the entry streams of a GinScanKey in a binary heap, to quickly
find the minimum curItem among them.

I did this in various versions of the fast scan patch, but then I
realized that the straightforward way of doing it is wrong, because a
single GinScanEntry can be part of multiple GinScanKeys. If an entry's
curItem is updated as part of advancing one key, and the entry is in a
heap of another key, updating the curItem can violate the heap property
of the other entry's heap.

* Build a truth table (or cache) of consistent-function's results, and
use that instead of calling consistent for every item.

* Deduce AND or OR logic from the consistent function. Or have the
opclass provide a tree of AND/OR/NOT nodes directly, instead of a
consistent function. For example, if the query is "foo & bar", we could
avoid calling consistent function altogether, and only return items that
match both.

* Delay decoding segments during a scan. Currently, we decode all
segments of a posting tree page into a single array at once. But with
"fast scan", we might be able to skip over all entries in some of the
segments. So it would be better to copy the segments into
backend-private memory in compressed format, and decode them one segment
at a time (or maybe even one item at a time), when needed. That would
avoid the unnecessary decoding of segments that can be skipped over, and
would also reduce memory usage of a scan.

I'll add these to the TODO.

- Heikki