channeldb+graph/db: move net.Addr encode/decode to graph db package

In preparation for moving the graph related schema structs to the graph
package in an upcoming commit, we move these methods to the graph
package. The structs we will move make use of these methods but we still
import them from channeldb so as to keep the ReadElement and
WriteElement helpers working as they do today.

graph/db/errors.go

@@ -0,0 +1,9 @@
+package graphdb
+
+import "fmt"
+
+var (
+	// ErrUnknownAddressType is returned when a node's addressType is not
+	// an expected value.
+	ErrUnknownAddressType = fmt.Errorf("address type cannot be resolved")
+)