iterate on close connections which violate policy after updates

[ilrudie]

• improved test coverage needed
• investigate if locking can be reduced in connection manager
  • use a different channel type, preferably one which exposes a reference count
  • use a different data structure instead of Arc<RwLock<HashMap<...>>>>, maybe (DashMap)
• replace the current release functionality with Drop to reduce likelihood of misuse/leaks

related #772 and #311

[howardjohn]

Consider using Drop for release to prevent any ability to miss it

[ilrudie]

#939 adds drop semantics to handle release

[ilrudie]

Ben has proposed using flurry in #931, probably use that instead of adding a second concurrent map crate unless there's a really good reason.