FileStorage: lockless reads and atomic writes

  - Reads don't lock anymore. At all.
  - Writes create a temporary file and use `rename(2)` to atomically
    replace it
  - since OpenOptions::create_new(true) is used, tempfile creation is
    atomic (and since tempfile names are per-post, a post can only be
    edited by one request at a time)
  - Since written files get atomically replaced, readers can't read a
    corrupted file

Potential pitfalls:
1. This approach is not covered by unit tests (yet)
2. Stale tempfiles can prevent editing posts (can be solved by
throwing out tempfiles that are older than, say, a day)
3. Crashed edits can leave stale tempfiles (honestly that sounds
better than corrupting the whole database, doesn't sound like a bug to
me at all!)

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