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

[Robert Collins]

Robert Collins has voted tweak.
Status is now: Conditionally approved
Comment:
Hmm, I may need to digest this. Re: the change to the builder - no 
problems there. It would be ideal though, if you could arrange it, to 
have a hint to the builder. pack and autopack definitely won't be 
reading back, commit however may be - so lets look at:
  - being able to never combine backing store
  - maintaining the backing store with a count cap that is a function of 
nodes-so-far. There is no point combining two 100K node indices to avoid 
searches. But there is a point in combining 10 - one 1M node index is 
what 3 levels vs 2 levels, so 20 queries vs 3.

As for random_id, it doesn't matter if it would collide with a different 
pack - that won't be picked up today anyway because we have a new-pack 
vf. As long as we won't emit the same id twice ourselves it will always 
be fine.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C49C2B2FA.4050208%40arbash-meinel.com%3E
Project: Bazaar