SSE implementation that sheds stuck clients (#14413)

* sse implementation that sheds stuck clients

* Radek and James feedback

* Refactor event streamer code for readability

* less-flaky test signaling

* test case where queue fills; fixes

* add changelog entry

* james and preston feedback

* swap our Subscription interface with an alias

* event.Data can be nil for the payload attr event

* deepsource

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>

async/event/interface.go

@@ -0,0 +1,8 @@
+package event
+
+// SubscriberSender is an abstract representation of an *event.Feed
+// to use in describing types that accept or return an *event.Feed.
+type SubscriberSender interface {
+	Subscribe(channel interface{}) Subscription
+	Send(value interface{}) (nsent int)
+}