* Remove unused backend wrapper from Cache
* Remove double printShutdownStatus
* Fix readyz race condition
* Test coverage for the readyz.monitor fix
* Close listeners immediately in proxy.shutdown
* Use and handle net.ErrClosed correctly
This adapts utils.IsUseOfClosedNetworkError to check for net.ErrClosed
even inside trace.Aggregate errors, makes it so that we always return
something that would pass errors.Is(err, net.ErrClosed) when returning
from a (net.Listener).Accept(), and handles closed listeners within our
various Serve() loops so that we don't hit spurious backoff waits while
shutting down.
* Close listeners early and emitters late
* Test coverage for the proxy listener changes
* Revert some errors back to trace.ConnectionProblem
* Reduce PR scope to just the proxy, add comments
* Improve error logging.
Edoardo Spadolini