QPID-4462: simply return the PlainSaslServer directly instead of looking up it, bypassing the lack of previous registration

Fix suggested by Alex Rudyy


git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1463063 13f79535-47bb-0310-9956-ffa450edef68
QPID-4677: group the constants for clarity and rename remaining inherited queue attribute in VirtualHost interface


git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1463064 13f79535-47bb-0310-9956-ffa450edef68