Initial UDT implementation for file transfer.

New function RequestFileTransferUDT to download files from other peers.

udt/duration_array.go

@@ -0,0 +1,17 @@
+package udt
+
+import "time"
+
+type sortableDurnArray []time.Duration
+
+func (a sortableDurnArray) Len() int {
+	return len(a)
+}
+
+func (a sortableDurnArray) Less(i, j int) bool {
+	return a[i] < a[j]
+}
+
+func (a sortableDurnArray) Swap(i, j int) {
+	a[i], a[j] = a[j], a[i]
+}