Don't check all realtime queues when doing "queue show some_queue".

When using realtime queues, queues have to be fetched from the database
every now and then to see if any info has been changed or to see if the
queue has been removed. When fetching info for an individual queue, the
pruning of other queues is unnecessarily costly.

Review: https://reviewboard.asterisk.org/r/2907/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@401049 f38db490-d61c-443f-a65b-d21fe96a405b