Refactor: streamline sasl auth

Let the SASL AUTH/STEP code follow the generic packet
handling code.

Change-Id: If26df8e326a016a83c7b4e4ca230fc3f07b38a6e
Reviewed-on: http://review.couchbase.org/42709
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: buildbot <build@couchbase.com>