Add option to use telegram test dcs

mtglib/internal/telegram/address_pool.go

@@ -0,0 +1,33 @@
+package telegram
+
+import "math/rand"
+
+type addressPool struct {
+	v4 [][]tgAddr
+	v6 [][]tgAddr
+}
+
+func (a addressPool) getV4(dc int) []tgAddr {
+	return a.get(a.v4, dc-1)
+}
+
+func (a addressPool) getV6(dc int) []tgAddr {
+	return a.get(a.v6, dc-1)
+}
+
+func (a addressPool) get(addresses [][]tgAddr, dc int) []tgAddr {
+	if dc < 0 || dc >= len(addresses) {
+		return nil
+	}
+
+	rv := make([]tgAddr, len(addresses[dc]))
+	copy(rv, addresses[dc])
+
+	if len(rv) > 1 {
+		rand.Shuffle(len(rv), func(i, j int) {
+			rv[i], rv[j] = rv[j], rv[i]
+		})
+	}
+
+	return rv
+}