plugin/pkg/cache: smarter locking (#1164)

Make the locking slightly smarter in Evict and add benchmark function.

Seems a bit faster (there was some variance while performing these
benchmarks)

Master:
BenchmarkCache-2   	 1000000	      2317 ns/op	       0 B/op	       0 allocs/op
BenchmarkCache-2   	 1000000	      2032 ns/op	       0 B/op	       0 allocs/op

This branch:
BenchmarkCache-2   	 1000000	      1806 ns/op	       0 B/op	       0 allocs/op
BenchmarkCache-2   	 1000000	      1809 ns/op	       0 B/op	       0 allocs/op

plugin/pkg/cache/cache_test.go

@@ -29,3 +29,13 @@ func TestCacheLen(t *testing.T) {
 		t.Fatalf("Cache size should %d, got %d", 2, l)
 	}
 }
+
+func BenchmarkCache(b *testing.B) {
+	b.ReportAllocs()
+
+	c := New(4)
+	for n := 0; n < b.N; n++ {
+		c.Add(1, 1)
+		c.Get(1)
+	}
+}