Index each cache node separately

Index each cache node separately

Background:

- Fort shouldn't lose the cache index when a signal interrupts it.
- Writing the index during the signal handler is not possible,
  because of the async-signal-safe requirement.
- Writing the index outside of the signal handler is seemingly not
  viable, because of the infelicities between the signal and
  multithreading APIs in C.

I haven't completely discarded the "dropping multithreading" option,
but since it seems disproportionate, I've been rethinking the index.

This commit scatters the index across several files, to minimize lost
information during a stopping signal. This will exacerbate the inode
problem, but that's temporary.

Reverts e0880f8eac3d35e576a2c1a2cb588424ab4b3e1b.