Better `modified` detection

[deforestg]

recursively convert dicts into CallbackDicts to better detect when values are changed

[deforestg]

decided against this

[fengsp]

@deforestg Why? Performance?

[deforestg]

because I realized that it inherently modifies the data that is being stored in the session if it is a dict - so for instance if you have a line that says if type(session['thing']) is dict: then that will no longer work since its been converted to a CallbackDict instead

[fengsp]

@deforestg And it doesn't apply to other mutable objects like list, so modified detection is still a problem. Try to keep it simple, just do this:

data = session['key']
# modify data here
session['key'] = data