sqldb: demonstrate the use of ExecutePagedQuery

Here, a new query (GetChannelsByOutpoints) is added which makes use of
the /*SLICE:outpoints*/ directive & added workaround. This is then used
in a test to demonstrate how the ExecutePagedQuery helper can be used to
wrap a query like this such that calls are done in pages.

The query that has been added will also be used by live code paths in an
upcoming commit.

sqldb/sqlc/queries/graph.sql

@@ -231,6 +231,17 @@ WHERE scid >= @start_scid
 SELECT * FROM graph_channels
 WHERE scid = $1 AND version = $2;
 
+-- name: GetChannelsByOutpoints :many
+SELECT
+    sqlc.embed(c),
+    n1.pub_key AS node1_pubkey,
+    n2.pub_key AS node2_pubkey
+FROM graph_channels c
+    JOIN graph_nodes n1 ON c.node_id_1 = n1.id
+    JOIN graph_nodes n2 ON c.node_id_2 = n2.id
+WHERE c.outpoint IN
+    (sqlc.slice('outpoints')/*SLICE:outpoints*/);
+
 -- name: GetChannelByOutpoint :one
 SELECT
     sqlc.embed(c),