Use NewMsgInvSizeHint when we know the size.

When trickling inv's to peers, allocate for the number of inv's
in the queue instead of the default 1000.  This should save on
memory.

peer.go

@@ -1670,7 +1670,7 @@ out:
 
 			// Create and send as many inv messages as needed to
 			// drain the inventory send queue.
-			invMsg := wire.NewMsgInv()
+			invMsg := wire.NewMsgInvSizeHint(uint(invSendQueue.Len()))
 			for e := invSendQueue.Front(); e != nil; e = invSendQueue.Front() {
 				iv := invSendQueue.Remove(e).(*wire.InvVect)
 
@@ -1685,7 +1685,7 @@ out:
 					waiting = queuePacket(
 						outMsg{msg: invMsg},
 						pendingMsgs, waiting)
-					invMsg = wire.NewMsgInv()
+					invMsg = wire.NewMsgInvSizeHint(uint(invSendQueue.Len()))
 				}
 
 				// Add the inventory that is being relayed to