[release-0.16] 🏃 Make client.MatchingLabels faster

The 99% use-case of this is to set a selector, not to adjust an existing
one. This change introduces a fastpath that does that with half the
allocations and in a bit less than half the time.

The reason slowpath is slow is that for each label a requirement has to
be constructed that is then appended to a slice, both of which cause
allocations.

pkg/client/options.go

@@ -514,7 +514,8 @@ type MatchingLabels map[string]string
 func (m MatchingLabels) ApplyToList(opts *ListOptions) {
 	// TODO(directxman12): can we avoid reserializing this over and over?
 	if opts.LabelSelector == nil {
-		opts.LabelSelector = labels.NewSelector()
+		opts.LabelSelector = labels.SelectorFromValidatedSet(map[string]string(m))
+		return
 	}
 	// If there's already a selector, we need to AND the two together.
 	noValidSel := labels.SelectorFromValidatedSet(map[string]string(m))