kvserver/rangefeed: Clear rather than Close memory budget

Clear is similar to Close but also resets the "used" and "reserved"
variables. This shouldn't really matter, but recent changes mean that
sometimes a budget for a processor might be "closed" before a give
registration using that processor's goroutine has finished
draining. This is OK since the feed budget is protected by a mutex and
allocations releases after close are allowed. However, it did break a
test under stress.

Epic: none
Release note: None

pkg/kv/kvserver/rangefeed/budget.go

@@ -228,7 +228,7 @@ func (f *FeedBudget) Close(ctx context.Context) {
 	f.closed.Do(func() {
 		f.mu.Lock()
 		f.mu.closed = true
-		f.mu.memBudget.Close(ctx)
+		f.mu.memBudget.Clear(ctx)
 		close(f.stopC)
 		f.mu.Unlock()
 	})