Currently the Kerberos auth mechanism and conversation live in the auxiliary Kerberos library, but their code is the same as code in other auth mechanisms. This causes issues when the driver is refactored as the Kerberos code becomes outdated with respect to the rest of the driver.

As part of this work the allowance for connection_description in Mongo::Operation::Result to be nil should be removed.

This change will also improve our Kerberos testing situation as all driver-related code will now be in the driver.

The code in question is all private APIs, hence there should not be direct impact on applications. However, the mechanism and conversation code must be removed from kerberos library (otherwise that code will overwrite whatever is in the driver), and this removal will require a major version bump for the kerberos library.

The actual MRI and JRuby extensions will remain in the Kerberos library for the foreseeable future. In particular it should continue to be possible to test these extensions with unit tests in the Kerberos library, though we presently don't have meaningful unit test coverage for them.