Allow specifying alternative connection_class

[davidlatwe]

This addresses #1168 (an old issue 😄).

This pull request opens door for projects like montydb or mongita that provids mongo-like alternative for testing or simple production usage, able to gain benefit from mongoengine.

There's some small amount of discussions in said projects:
scottrogowski/mongita#4
davidlatwe/montydb#64

And the only workaround for this is to monkey-patch pymongo.MongoClient just like what #1168 has concluded.

But thanks to the evolution that mongoengine has been through, the codebase seems able to enable this feature easily. Hance this PR.

Please let me know what you think, thanks.

[bagerard]

This is great and is much more elegant and less invasive than the previous way to integrate mongomock. Thanks for the contribution 🚀

I did some modification on top of it here, should be merged soon