Use `Arc<[Arc<BridgeConfig>]>` in configuration, GuardMgr, etc.

We frequently want to pass around an Arc<BridgeConfig>, such as when we are making a BridgeRelay, or when we're telling the BridgeDescProvider what bridge we want.

But right now, GuardMgrConfig (and hence a lot of other stuff) give us a Vec<BridgeConfig>, which GuardMgr turns into an Arc<[BridgeConfig]>. We should introduce another internal Arc.

See discussion at !847 (comment 2853577)