Fix head chunk persisting and a chunkDesc race condition.

- Head chunk persisting only happens in evictOlderThan, so do it
  there.  (With the previous code, it would never happen.)

- Raw accesses to chunkDesc.chunk are now done via isEvicted (with
  locking).

Change-Id: I48b07b56dfea4899b50df159b4ea566954396fcd