refactor: move graph related DB code to graph/db from channeldb

This is a pure refactor commit. It moves over all the graph related CRUD
code from `channeldb` to `graph/db`.

discovery/syncer.go

@@ -11,8 +11,8 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
-	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/graph"
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"golang.org/x/time/rate"
@@ -373,7 +373,7 @@ type GossipSyncer struct {
 
 	// bufferedChanRangeReplies is used in the waitingQueryChanReply to
 	// buffer all the chunked response to our query.
-	bufferedChanRangeReplies []channeldb.ChannelUpdateInfo
+	bufferedChanRangeReplies []graphdb.ChannelUpdateInfo
 
 	// numChanRangeRepliesRcvd is used to track the number of replies
 	// received as part of a QueryChannelRange. This field is primarily used
@@ -837,7 +837,7 @@ func (g *GossipSyncer) processChanRangeReply(msg *lnwire.ReplyChannelRange) erro
 	g.prevReplyChannelRange = msg
 
 	for i, scid := range msg.ShortChanIDs {
-		info := channeldb.NewChannelUpdateInfo(
+		info := graphdb.NewChannelUpdateInfo(
 			scid, time.Time{}, time.Time{},
 		)
 
@@ -1115,7 +1115,7 @@ func (g *GossipSyncer) replyChanRangeQuery(query *lnwire.QueryChannelRange) erro
 	// this as there's a transport message size limit which we'll need to
 	// adhere to. We also need to make sure all of our replies cover the
 	// expected range of the query.
-	sendReplyForChunk := func(channelChunk []channeldb.ChannelUpdateInfo,
+	sendReplyForChunk := func(channelChunk []graphdb.ChannelUpdateInfo,
 		firstHeight, lastHeight uint32, finalChunk bool) error {
 
 		// The number of blocks contained in the current chunk (the
@@ -1164,7 +1164,7 @@ func (g *GossipSyncer) replyChanRangeQuery(query *lnwire.QueryChannelRange) erro
 	var (
 		firstHeight  = query.FirstBlockHeight
 		lastHeight   uint32
-		channelChunk []channeldb.ChannelUpdateInfo
+		channelChunk []graphdb.ChannelUpdateInfo
 	)
 
 	// chunkSize is the maximum number of SCIDs that we can safely put in a