Fix read_preference

[wojcikstefan]

I noticed something disturbing in our query logs earlier today - some of the queries were incorrectly routed to the primary when using ReadPreference.SECONDARY. Eventually, it came to my realization that read_preference wasn't working if it was put after limit, skip, or hint (and probably some other methods that allow chaining).

Note: As of now, the fix only works for pymongo ver < 3.0. I think read_preference is completely broken with ver >= 3.0, but I don't have full clarity on the situation yet. It seems that self._collection.find doesn't accept read_preference as a valid kwarg any more and if I'm reading pymongo's docs right, you can only set read preference on a client/database/collection, not a particular cursor. It doesn't seem to be set either way:

In [10]: import mongoengine

In [11]: from mongoengine import *

In [12]: from pymongo.read_preferences import ReadPreference

In [13]: import pymongo

In [14]: pymongo.version
Out[14]: '3.0.2'

In [15]: class A(Document):
    txt = StringField()
   ....:

In [16]: qs = A.objects.read_preference(ReadPreference.SECONDARY)

In [17]: qs._cursor._Cursor__read_preference
Out[17]: Primary()

In [18]: qs._cursor.collection.read_preference
Out[18]: Primary()

[landscape-bot]

Repository health increased by 0.01% when pulling 3f80337 on elasticsales:fix-read-preference into c6151e3 on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[MRigal]

Yes, it works quite differently with PyMongo3, it has to be defined at connection. We need at least to update the docs and also maybe to raise some additional warning.

In order to make your PR mergeable, please add an if for the assert statement if IS_PYMONGO3 adding a comment why it is not working anymore with PyMongo3

[landscape-bot]

Repository health decreased by 0.00% when pulling bf169ac on elasticsales:fix-read-preference into c6151e3 on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[wojcikstefan]

@MRigal I rebased this PR against the latest master and also fixed read_preference for PyMongo 3+. Let me know what you think.

[landscape-bot]

Repository health decreased by 0.00% when pulling 8b84014 on elasticsales:fix-read-preference into 9671ca5 on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• No problems were fixed.

[wojcikstefan]

@thedrow rebased.

[amcgregor]

@thedrow Ref #1169 could we get a Python Package Index release to go with the Git tags?