Update PersistentDict and create WriteOncePersistentDict.

Major changes to PersistentDict:
  * Uses a per-file lock instead of a per-container-dir lock.
  * Removed info files from store().
  * Checks for cache collision on delete.
  * Added more tests.

This change also implements WriteOncePersistentDict, which has a
write-once policy.  WriteOncePersistentDict uses a writer lock but
allows for unlocked reads, which makes reads from disk faster.  It
also uses an in-memory LRU cache to speed up accesses.