Write out to the backing store asynchronously.
Despite the fact AmigaOS filesystems run as separate processes,
DOS filesystem writes occur synchronously waiting for the disk
write to finish before returning.  Here we have effectively taken
the core's fs backing store and extended it slightly so the store
operation is spun off into a new process, running at a lower task
priority than NetSurf itself.  This stops the UI pauses that occur
shortly after loading pages as NetSurf commits the cache to disk.

Somewhat experimental, pending more extensive user testing.
make backing store use rename to atomicly replace entries file