[MERGE] Set allow_optimize=False when spilling btree content to disk

[John Arbash Meinel]

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The attached patch changes the btree code slightly when spilling indices
to disk to avoid memory pressure.

Prior to this, if you called "set_optimize(for_size=True)" (as we do
during 'bzr pack', and as the groupcompress does for 'autopack' as
well), it would create all indexes in "maximum packed" form.

However, when spilling to disk, we only do that to reduce memory
pressure, since we are going to come back and rebuild it into a new
index anyway.

It doesn't have any tests, because we don't really have testing of how
the "_optimize_for_size" gets passed around. The details seem a bit
hidden, and the amount of time to rewrite the code to get it testable
seemed a rather high barrier to entry.

I'm sending a follow up to this, and when combining the two bits, it
shaves ~1m / 11m on 'bzr pack' of a launchpad tree.

John
=:->
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