kvstreamer: remove a memory leak

At the moment, we have a memory leak of `Streamer` objects (although
nil-ed out) because of `SetOnChange` handler of the streamer concurrency
limit cluster setting and passing in a closure into `Stopper.AddCloser`.
This was copied over from the `DistSender` code, but a crucial difference
wasn't appreciated - we have a single global `DistSender` that lives
throughout the uptime of the server whereas each `Streamer` object lives
only during the query execution.

We don't need to dynamically react to changes in the streamer concurrency
limits, so this commit removes the handler. The closure has been
refactored too.

Release note: None

pkg/kv/kvclient/kvstreamer/streamer.go

@@ -335,10 +335,6 @@ func NewStreamer(
 		"single Streamer async concurrency",
 		uint64(streamerConcurrencyLimit.Get(&st.SV)),
 	)
-	streamerConcurrencyLimit.SetOnChange(&st.SV, func(ctx context.Context) {
-		s.coordinator.asyncSem.UpdateCapacity(uint64(streamerConcurrencyLimit.Get(&st.SV)))
-	})
-	stopper.AddCloser(s.coordinator.asyncSem.Closer("stopper"))
 	return s
 }
 
@@ -398,7 +394,7 @@ func (s *Streamer) Enqueue(
 ) (retErr error) {
 	if !s.coordinatorStarted {
 		var coordinatorCtx context.Context
-		coordinatorCtx, s.coordinatorCtxCancel = context.WithCancel(ctx)
+		coordinatorCtx, s.coordinatorCtxCancel = s.stopper.WithCancelOnQuiesce(ctx)
 		s.waitGroup.Add(1)
 		if err := s.stopper.RunAsyncTaskEx(
 			coordinatorCtx,