[WFLY-4184] :list-delivering-messages operation

add the list-delivering-messages (and
list-delivering-messages-as-json) operation to the messaging's queue
(and runtime-queue) resources to list the messages that are currently
delivered for a given queue and which consumers is receiving them.

JIRA: https://issues.jboss.org/browse/WFLY-4184
Merge pull request #7045 from jmesnil/WFLY-4184_list-delivering-messages

[WFLY-4184] :list-delivering-messages operation