Access to long-unsettled (stuck) deliveries

Implement a remote-management access mechanism to find, enumerate, and query deliveries that have been unsettled in the router for a long time (i.e. > 10 seconds).

Such deliveries will not be counted in the statistics introduced in ENTMQIC-2123 because they are not yet settled. This is a separate mechanism to:

• Discover when stuck deliveries exist in the router
• Enumerate the stuck deliveries so it can be determined which links and connections are involved (both ingress and egress)
• Query individual stuck deliveries to retrieve their headers and any other pertinent metadata