cmd/bloom: map only one .idx file at a time.
cmd/bloom: map only one .idx file at a time.

This massively decreases virtual memory allocation since we only ever need
to look at a single idx at once.

In theory, VM doesn't cost us anything, but on 32-bit systems we can
actually run out of address space if we try to map all the idx files at
once on a very large repo.

Signed-off-by: Avery Pennarun <e03ccfb89d416c4697ad6970ce86a92c1c28a78e@gmail.com>

gevaerts/bup,pombredanne/bup,apenwarr/bup,jbaber/bup,apenwarr/bup,mhoeher/bup,pombredanne/bup,ToxicFrog/bup,apenwarr/bup,jbaber/bup,ToxicFrog/bup,tjanez/bup,pombredanne/bup,jbaber/bup,pombredanne/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,tjanez/bup,ToxicFrog/bup,mhoeher/bup,tjanez/bup,ToxicFrog/bup,mhoeher/bup,mhoeher/bup,jbaber/bup,ToxicFrog/bup,tjanez/bup,mhoeher/bup,gevaerts/bup,gevaerts/bup,jbaber/bup,jbaber/bup,pombredanne/bup,gevaerts/bup,gevaerts/bup,tjanez/bup,apenwarr/bup