Switch peer.requestQueue from linked list to slice.

Only two operations are performed with this data structure: adding to
the back and removing from the front.  Because middle inserts and
deletions are never needed, a linked list results in overall worse
performance due to an extra allocation for each element's node, worse
cache locality, and the runtime cost of boxing/unboxing each item
during accesses.

On top of the performance gains, a slice is more type safe as it is a
true generic data structure making it is impossible to insert or
access an element with the wrong type.

blockmanager.go

@@ -930,7 +930,7 @@ func (b *blockManager) handleInvMsg(imsg *invMsg) {
 		}
 		if !haveInv {
 			// Add it to the request queue.
-			imsg.peer.requestQueue.PushBack(iv)
+			imsg.peer.requestQueue = append(imsg.peer.requestQueue, iv)
 			continue
 		}
 
@@ -980,9 +980,10 @@ func (b *blockManager) handleInvMsg(imsg *invMsg) {
 	numRequested := 0
 	gdmsg := btcwire.NewMsgGetData()
 	requestQueue := imsg.peer.requestQueue
-	for e := requestQueue.Front(); e != nil; e = requestQueue.Front() {
+	for len(requestQueue) != 0 {
-		iv := e.Value.(*btcwire.InvVect)
+		iv := requestQueue[0]
-		imsg.peer.requestQueue.Remove(e)
+		requestQueue[0] = nil
+		requestQueue = requestQueue[1:]
 
 		switch iv.Type {
 		case btcwire.InvTypeBlock:
@@ -1010,6 +1011,7 @@ func (b *blockManager) handleInvMsg(imsg *invMsg) {
 			break
 		}
 	}
+	imsg.peer.requestQueue = requestQueue
 	if len(gdmsg.InvList) > 0 {
 		imsg.peer.QueueMessage(gdmsg, nil)
 	}

peer.go

@@ -162,7 +162,7 @@ type peer struct {
 	prevGetBlocksStop  *btcwire.ShaHash // owned by blockmanager
 	prevGetHdrsBegin   *btcwire.ShaHash // owned by blockmanager
 	prevGetHdrsStop    *btcwire.ShaHash // owned by blockmanager
-	requestQueue       *list.List
+	requestQueue       []*btcwire.InvVect
 	filter             *bloom.Filter
 	relayMtx           sync.Mutex
 	disableRelayTx     bool
@@ -1890,7 +1890,6 @@ func newPeerBase(s *server, inbound bool) *peer {
 		knownInventory:  NewMruInventoryMap(maxKnownInventory),
 		requestedTxns:   make(map[btcwire.ShaHash]struct{}),
 		requestedBlocks: make(map[btcwire.ShaHash]struct{}),
-		requestQueue:    list.New(),
 		filter:          bloom.LoadFilter(nil),
 		outputQueue:     make(chan outMsg, outputBufferSize),
 		sendQueue:       make(chan outMsg, 1),   // nonblocking sync