Use a random subset of vectors to generate trees

[Kerollmops]

The current indexer uses the full set of vectors to generate the trees. The advantage is that the split planes are very representative of the actual vectors we have in the database. The const is that it takes a lot of memory, even if it is memory-mapped.

So, to reduce the memory used to index the vectors, we should only use a subset of the vectors to generate the tree and then add the other vectors, the ones not used to generate the planes, back into the tree. With or without refining the planes, I don't know, but probably without. Those vectors can probably be added by using incremental insertion.