JCBC-346: Automatically detect SASL mechanisms.

This changeset, together with the corresponding spymemcached
changeset makes sure that the server automatically gets asked
for all supported SASL mechanisms and the most secure one is
selected.

A specific mechanism can be forced by providing a custom
AuthDescriptor.

Change-Id: I27b5ace1dc884f370f14db8ba713221a5e8779fe
Reviewed-on: http://review.couchbase.org/28389
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>