graph/db+sqldb: make policy migration idempotent

Finally, we make the channel-policy part of the SQL migration idempotent
by adding a migration-only policy insert query which will not error out
if the policy already exists and does not have a timestamp that is newer
than the existing records timestamp. To keep the commit simple, a
insertChanEdgePolicyMig function is added which is basically identical
to the updateChanEdgePolicy function except for the fact that it uses
the newly added query. In the next commit, it will be simplified even
more.

graph/db/sql_store.go

@@ -164,6 +164,7 @@ type SQLQueries interface {
 	*/
 	InsertNodeMig(ctx context.Context, arg sqlc.InsertNodeMigParams) (int64, error)
 	InsertChannelMig(ctx context.Context, arg sqlc.InsertChannelMigParams) (int64, error)
+	InsertEdgePolicyMig(ctx context.Context, arg sqlc.InsertEdgePolicyMigParams) (int64, error)
 }
 
 // BatchedSQLQueries is a version of SQLQueries that's capable of batched