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

UniqueNameGenerator: Add a _name_added() callback.

See merge request !2

Prep for Flake8 during CI

See merge request !1

[bugfix] Use base classes for ImmutableRecord in correct order

A very weird bug: The order of base classes actually matters here,
because if __hash__ and __eq__ are defined in the wrong order,
__hash__ will get deleted (in 3). MWE:

```
class X(object):
    def __eq__(self, other):
        return True

class Y(object):
    def __hash__(self):
        return 0

class Z1(Y,X):
    pass

class Z2(X,Y):
    pass

hash(Z1())
hash(Z2())
```