Python driver should not allow sending _ids with a key starting with $

See related SERVER-12209.

The Python driver checks other document keys and disallows prefixed $, but apparently not for _id.

Any such _id gets inserted on the primary (despite --objcheck) but will crash all secondaries immediately after.

The Python driver should reject sending such bad {{_id}}s, and additionally converting the obvious {$oid: "..."} form to a proper ObjectId type instead.

This code run against a replica set primary crashes all secondaries:

from pymongo import MongoClient

mc = MongoClient()

mc.test.docs.insert({'_id': {'$oid': "52d0b971b3ba219fdeb4170e"}})