channeldb: embed the instance of boltb within DB struct

This commit modifies the composition of the boltdb pointer within the
DB struct to use embedding.

The rationale for this change is that the daemon may soon store some
semi-transient items within the database which requires us to expose
the boltdb’s transaction API. The logic for serialization of this data
will likely lie outside of the channeldb package as the items that may
be stored in the future will be specific to the current sub-systems
within the daemon and not generic channel related data.

channeldb/nodes.go

@@ -104,7 +104,7 @@ func (l *LinkNode) Sync() error {
 
 	// Finally update the database by storing the link node and updating
 	// any relevant indexes.
-	return l.db.store.Update(func(tx *bolt.Tx) error {
+	return l.db.Update(func(tx *bolt.Tx) error {
 		nodeMetaBucket := tx.Bucket(nodeInfoBucket)
 		if nodeInfoBucket == nil {
 			return fmt.Errorf("node bucket not created")
@@ -139,7 +139,7 @@ func (db *DB) FetchLinkNode(identity *btcec.PublicKey) (*LinkNode, error) {
 		err  error
 	)
 
-	err = db.store.View(func(tx *bolt.Tx) error {
+	err = db.View(func(tx *bolt.Tx) error {
 		// First fetch the bucket for storing node meta-data, bailing
 		// out early if it hasn't been created yet.
 		nodeMetaBucket := tx.Bucket(nodeInfoBucket)
@@ -178,7 +178,7 @@ func (db *DB) FetchLinkNode(identity *btcec.PublicKey) (*LinkNode, error) {
 func (db *DB) FetchAllLinkNodes() ([]*LinkNode, error) {
 	var linkNodes []*LinkNode
 
-	err := db.store.View(func(tx *bolt.Tx) error {
+	err := db.View(func(tx *bolt.Tx) error {
 		nodeMetaBucket := tx.Bucket(nodeInfoBucket)
 		if nodeInfoBucket == nil {
 			return fmt.Errorf("node bucket not created")