PYTHON-737 Raise AutoReconnect on exhaust cursor error.

Cursor had raised socket.error when failing to receive a batch from an exhaust
cursor, but it should raise AutoReconnect.

Also forward-port exhaust cursor tests from v2.8 to 3.0-dev.
Remove _TestLazyConnectMixin class.

This was necessary when we tested MongoClient and MongoReplicaSetClient
separately in PyMongo 2.x. In PyMongo 3.x the classes are unified so we
test all lazy-connection functionality in the same test class in test_client.py.