Store if not present in persistent dicts

See merge request !5

PersistentDict cleanup; create WriteOncePersistentDict

See merge request inducer/pytools!4

Conflicts:
	test/test_persistent_dict.py

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.

the version read is for backwards compatibility.

This adds an optional in-memory LRU cache. To use the cache, you
supply a *in_mem_cache_size* parameter to the PersistentDict.

In order to properly support cache invalidation this also implements
version tracking of files. This change is backwards compatible with
existing persistent dict caches.

Nested saved persistent hashes

See merge request !3