uri: rename mechanism parameter to authMechanism.

This matches what the other drivers are doing for authentication
mechanism in SASL.
sasl: check for {done: 1} when authenticating against GSSAPI.

This gets SASL working for me on my test setup. We still need to abstract
this into it's own module and remove the goto's for control flow.

We probably need a maximum number of steps during sasl_client_step()
too.

Additionally, we need some work to verify thread safety.