CircMgr::reconfigure must look at bridge configuration

In earlier design discussions, we decided that we didn't want to close all our existing channels and circuits when we changed our bridges, or we changed from using bridges to not using bridges. That's cool.

We should, however, tell the CircMgr that the circuits it was previously using might not be usable any more:

• a non-bridge circuit is non-usable if we start using bridges.
• a bridge circuit is non-usable if we stop using bridges, or if we remove that bridge from our configuration.

We have some logic to handle a similar case already: see retire_all_circuits() and its use in CircMgr::reconfigure. See also the TODO(nickm) comment in retire_all_circuits().