Update the test suite

*) DONE: test MongoClient instead of Connection, except for a few specific tests to make sure Connection still works

*) DONE: get_client() instead of get_connection()

*) DONE: rename test_connection to test_mongo_client or test_client

*) DONE: rename TestConnection to TestMongoClient or TestClient

*) DONE: rename other stray identifiers that refer to 'connection' where they now mean 'client'

*) DONE: fix all the tests that rely on the old defaults for 'safe' and 'auto_start_request'