bridge desc mgr should be able to be inactive

We would like the BridgeDescMgr handle (i) whether-we-want-to-bootstrap and (ii) dormancy, so that even if guardmgr tells it a nonempty set of bridges, it might still do nothing. This will involve an extra method to say "you may start work now" (called from the toplevel bootstrap) and an extra parameter (the dormancy channel).

CC @nickm