Use string pool for file set symlinks

Use string pool for file set symlinks

- Removes the last use of our former simple, stupid and slow caches
- For now, use a per-fi pool for this just like the previous caching
  did. Memory use is slightly increased but its faster than before,
  to reap the full benefits (memory and otherwise) we'll want a
  per-transaction pool for these, to be added later.