support ability to clear IPC queues

[gordon chung]

we don't have rolling upgrades in that you can notification agents from different branches working together (this might work but we don't officially)

i have a proposal that will definitely break this so it'd be good to enable ability to disable listening to main queue while supporting ability to clear interim queues. this might be possible already by just removing all targets but i've never verified and this is just raising discussion whether we need explicit way to support this or just document disabling targets for upgrade