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`.

channeldb/channel_test.go

@@ -2,6 +2,7 @@ package channeldb
 
 import (
 	"bytes"
+	"encoding/hex"
 	"math/rand"
 	"net"
 	"reflect"
@@ -10,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
@@ -24,6 +26,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntest/channels"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/shachain"
 	"github.com/lightningnetwork/lnd/tlv"
 	"github.com/stretchr/testify/require"
@@ -43,8 +46,20 @@ var (
 	}
 	privKey, pubKey = btcec.PrivKeyFromBytes(key[:])
 
+	testRBytes, _ = hex.DecodeString("8ce2bc69281ce27da07e6683571319d18e" +
+		"949ddfa2965fb6caa1bf0314f882d7")
+	testSBytes, _ = hex.DecodeString("299105481d63e0f4bc2a88121167221b67" +
+		"00d72a0ead154c03be696a292d24ae")
+	testRScalar = new(btcec.ModNScalar)
+	testSScalar = new(btcec.ModNScalar)
+	_           = testRScalar.SetByteSlice(testRBytes)
+	_           = testSScalar.SetByteSlice(testSBytes)
+	testSig     = ecdsa.NewSignature(testRScalar, testSScalar)
+
 	wireSig, _ = lnwire.NewSigFromSignature(testSig)
 
+	testPub = route.Vertex{2, 202, 4}
+
 	testClock = clock.NewTestClock(testNow)
 
 	// defaultPendingHeight is the default height at which we set