Add information request list to DHT structure. It keeps track of requests and matches responses.

Break up SendRequest into individual functions SendRequestStore and SendRequestFindNode, which improves readability.
Alpha parameter must be now passed. MarkNodeAsSeen is exported.
Breaking up dht.iterate and embedding into 3 functions Get, FindNode, and FindValue.

dht/Node.go

@@ -127,10 +127,11 @@ func (n *shortList) GetUncontacted(count int, useCount bool) (Nodes []*Node) {
 
 // NodeMessage is a message sent by a node
 type NodeMessage struct {
-	SenderID []byte // Sender of this message
-	Data     []byte
-	Closest  []*Node
-	Error    error
+	SenderID []byte  // Sender of the message
+	Data     []byte  // FIND_VALUE: Actual data
+	Closest  []*Node // FIND_VALUE, FIND_NODE: Closest nodes to the requested key
+	Storing  []*Node // FIND_VALUE: Nodes known to store the value
+	Error    error   // To be removed
 }
 
 // NodeFilterFunc is called to filter nodes based on the callers choice