sqldb/sqlc: add zombie index table

Note that this table will only contain entries for channels that we have
deleted from the `channels` table which is why we cannot use foreign
keys. Similarly, we may no longer have node entries for the nodes in the
table.

sqldb/sqlc/models.go

@@ -184,3 +184,10 @@ type NodeFeature struct {
 type SourceNode struct {
 	NodeID int64
 }
+
+type ZombieChannel struct {
+	Scid     []byte
+	Version  int16
+	NodeKey1 []byte
+	NodeKey2 []byte
+}