Problem handling dots in DictField keys

Consider the following code:

class TestModel(db.Document):
    test = db.DictField()

obj = TestModel()
obj.test['key.with.dot'] = 'abc'
obj.save()

I think this should work, because MongoDB allows dots in dict key names as of version 3.6.
But in fact I get the following error:

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    obj.save()
  File "mongoengine/document.py", line 450, in save
    self._clear_changed_fields()
  File "mongoengine/base/document.py", line 547, in _clear_changed_fields
    field_name = data._reverse_db_field_map.get(part, part)
AttributeError: 'NoneType' object has no attribute '_reverse_db_field_map'

After some digging I found that _clear_changed_fields takes list of string keys from _get_changed_fields() where each key is a dot-delimited path into the object. Obviously this doesn't play well with keys containing dots.

I think it would be safer to store keys as tuples rather than dot-delimited strings, but it will probably involve too many changes?

Tested with MongoEngine==0.23.1 and python 3.9.5.

Start with the provided TestModel reproduction and trace _get_changed_fields() into _clear_changed_fields() in mongoengine/base/document.py. Check how dot-delimited paths and DictField keys are represented, then verify that saving a key containing a dot completes without the reported AttributeError.