cmd/server: find .idx filenames more efficiently when needed.

cmd/server: find .idx filenames more efficiently when needed.

Rather than mapping *all* the .idx files into memory at once just to look up
a single object, just open/read/close them sequentially.  This should
significantly increase the total repo size on a 32-bit system.  (Of course,
it's still not very ideal; we really should have some kind of fallback mode
for when our total set of indexes starts getting too big.)

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