INTPYTHON-617 - Improve DictField to_python performance

[NoahStapp]

Improves performance of large documents with nested DictFields by nearly 20x, addressing #1230.

Modified benchmark script pulled from https://stackoverflow.com/questions/35257305/mongoengine-is-very-slow-on-large-documents-compared-to-native-pymongo-usage/:

import datetime
import itertools
import random
import timeit
from collections import defaultdict

import mongoengine as db

db.connect("test-dicts")

class MyModel(db.Document):
    date = db.DateTimeField(required=True, default=datetime.date.today)
    data_dict_1 = db.DictField(required=False)

MyModel.drop_collection()

data_1 = ['foo', 'bar']
data_2 = ['spam', 'eggs', 'ham']
data_3 = ["subf{}".format(f) for f in range(5)]

m = MyModel()
tree = lambda: defaultdict(tree)
data = tree()
for _d1, _d2, _d3 in itertools.product(data_1, data_2, data_3):
    data[_d1][_d2][_d3] = list(random.sample(range(50000), 20000))
m.data_dict_1 = data
m.save()

def pymongo_doc():
    return db.connection.get_connection()["test-dicts"]['my_model'].find_one()

def mongoengine_doc():
    model = MyModel.objects.first()
    return model

if __name__ == '__main__':
    print("pymongo took {:2.2f}s".format(timeit.timeit(pymongo_doc, number=10)))
    print("mongoengine took {:2.2f}s".format(timeit.timeit(mongoengine_doc, number=10)))

Before:

pymongo took 0.21s
mongoengine took 4.98s

After:

pymongo took 0.20s
mongoengine took 0.20s