Reduced startup time with large rock cache_dirs (#634)

Reduced startup time with large rock cache_dirs (#634)

... addressing an old TODO.

Before scanning the disks to find the actual entries, the old rock
cache_dir initialization code had to populate its index as if all disk
slots were available and then remove all the added entries. Those two
wasteful operations took ~1.5 seconds for a 200GB disk before the
PageStack ABA bug was fixed in a586085. With the tree-based fix, that
time increased to ~15 seconds. The delay is completely gone now,
reducing the total index initialization time (for a 200GB disk) down to
a second.