Log a warning for a router that is badly backed up

When the work queue for a router becomes very long, log
warnings as new items are added instead of debug messages.

Change-Id: I80c7107c369b7e9207a78b4e4c9147388c637ad2
Merge pull request #137 from dhellmann/warn-on-deep-queue

Log a warning for a router that is badly backed up