lnwire: separate ChannelID into a distinct struct, add tests

This commit separates out the ChannelID into a new file, with
additional helper methods for conversion and formatting. With this
commit, the struct is now more general purpose and can be used in the
new routing package, database, and other related sub-systems.

lnwire/channel_announcement.go

@@ -3,35 +3,13 @@ package lnwire
 import (
 	"bytes"
 	"fmt"
+	"io"
+
 	"github.com/go-errors/errors"
 	"github.com/roasbeef/btcd/btcec"
 	"github.com/roasbeef/btcd/wire"
-	"io"
 )
 
-// ChannelID represent the set of data which is needed to retrieve all
-// necessary data to validate the channel existance.
-type ChannelID struct {
-	// BlockHeight is the height of the block where funding
-	// transaction located.
-	// NOTE: This field is limited to 3 bytes.
-	BlockHeight uint32
-
-	// TxIndex is a position of funding transaction within a block.
-	// NOTE: This field is limited to 3 bytes.
-	TxIndex uint32
-
-	// TxPosition indicating transaction output which pays to the
-	// channel.
-	TxPosition uint16
-}
-
-func (c *ChannelID) String() string {
-	return fmt.Sprintf("BlockHeight:\t\t\t%v\n", c.BlockHeight) +
-		fmt.Sprintf("TxIndex:\t\t\t%v\n", c.TxIndex) +
-		fmt.Sprintf("TxPosition:\t\t\t%v\n", c.TxPosition)
-}
-
 // ChannelAnnouncement message is used to announce the existence of a channel
 // between two peers in the overlay, which is propagated by the discovery
 // service over broadcast handler.
@@ -200,7 +178,7 @@ func (c *ChannelAnnouncement) String() string {
 	return fmt.Sprintf("\n--- Begin ChannelAnnouncement ---\n") +
 		fmt.Sprintf("FirstNodeSig:\t\t%v\n", c.FirstNodeSig) +
 		fmt.Sprintf("SecondNodeSig:\t\t%v\n", c.SecondNodeSig) +
-		fmt.Sprintf("ChannelID:\t\t%v\n", c.ChannelID.String()) +
+		fmt.Sprintf("ChannelID:\t\t%v\n", c.ChannelID.ToUint64()) +
 		fmt.Sprintf("FirstBitcoinSig:\t\t%v\n", c.FirstBitcoinSig) +
 		fmt.Sprintf("SecondBitcoinSig:\t\t%v\n", c.SecondBitcoinSig) +
 		fmt.Sprintf("FirstNodeSig:\t\t%v\n", c.FirstNodeSig) +