This branch removes the fall-back-to-DNS behaviour from the proxy's destination module. Instead, when the Destination service returns "no endpoints", the proxy will route the request to its original destination, using the fallback behaviour introduced in #248. In addition, the proxy now no longer reconnects to the destination service for queries which receive a response with the Invalid Argument status code. After linkerd/linkerd2#2664, this status is used by the control plane to indicate names which should not query the destination service. I've added a new test for this. Furthermore (and unlike #245), this branch also removes the caching of service discovery lookups so that they can be shared between resolvers for authorities with the same canonical form. When we limited the number of concurrently in flight destination service queries to 100, reusing them was much more important; now it adds a lot of unnecessary complexity. The rewritten `control::destination` module is now much simpler and (hopefully) easier to understand. The `Resolver` holds a destination service `Client`, which is cloned every time a new `Resolution` is requested. Each active `Resolution` has a corresponding background task that drives the destination service query and sends updates through a channel to the `Resolver`. The background task is necessary so that updates are processed as they are recieved, rather than only when the `Resolution` is polled (i.e. when the load balancer is routing a request). In addition, the channel provides the queueing necessary to transform the control plane's update types (which contain multiple endpoint addresses) into multiple `tower-balance` updates for single endpoints, and to invalidate stale endpoints when the control plane connection is reestablished. When a control plane query is not necessary because the requested authority does not match the proxy's destination suffixes, the background task is not spawned and the `Resolution` remains permanently in the "no endpoints" state; and when an `InvalidArgument` error code is recieved, the background task terminates. A majority of the service discovery tests pass with no changes after this change. The following changes were necessary: * The test `outbound_updates_newer_services` was removed, as it tests that the same destination service query is shared between multiple services with the same authority (which is no longer the case). * The mock destination service was changed to send the `Unavailable` status code when it has run out of expected destinations, rather than `InvalidArgument`, as that code is used to indicate that a destination should never be reconnected. * New tests were added for falling back to ORIG_DST when no endpoints exist, and for not reconnecting after an `InvalidArgument` error. Closes #245. Closes linkerd/linkerd2#2661 Closes linkerd/linkerd2#2728 Signed-off-by: Eliza Weisman eliza@buoyant.io

proxy: Don't retry discovery requests that return InvalidArgument #2728

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