selector: new selector interface with random & roundrobin implementation (#1761)

* selector: implement new selector interface plus random & roundrobin implementations

* selector/roundrobin: remove unused consts

* router: add close method to interface

* selector/roundrobin: fix concurrent map iteration and map write

* selector: replace variadic argument on Select

selector/random/random_test.go

@@ -0,0 +1,11 @@
+package random
+
+import (
+	"testing"
+
+	"github.com/micro/go-micro/v2/selector"
+)
+
+func TestRandom(t *testing.T) {
+	selector.Tests(t, NewSelector())
+}