graph/db+sqldb: delete channels in batches

Use the new `SLICES` directive to add a DeleteChannels query which takes
a set of DB channel IDs. Then replace all our calls to DeleteChannel
with a paginated call to DeleteChannels.

sqldb/sqlc/querier.go

@@ -18,8 +18,8 @@ type Querier interface {
 	CreateChannel(ctx context.Context, arg CreateChannelParams) (int64, error)
 	CreateChannelExtraType(ctx context.Context, arg CreateChannelExtraTypeParams) error
 	DeleteCanceledInvoices(ctx context.Context) (sql.Result, error)
-	DeleteChannel(ctx context.Context, id int64) error
 	DeleteChannelPolicyExtraTypes(ctx context.Context, channelPolicyID int64) error
+	DeleteChannels(ctx context.Context, ids []int64) error
 	DeleteExtraNodeType(ctx context.Context, arg DeleteExtraNodeTypeParams) error
 	DeleteInvoice(ctx context.Context, arg DeleteInvoiceParams) (sql.Result, error)
 	DeleteNode(ctx context.Context, id int64) error