Refactor SASL auth handling

Add tests, use proper ByteBuffers

Change-Id: Ie62b909b534d3ceb8144eee5f2c2af87526545ef
Reviewed-on: http://review.couchbase.org/32617
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>