graph/db+sqldb: pass set of outpoints to SQL

This commit adds a new GetChannelsByOutpoints query which takes a slice
of outpoint strings. This lets us then update PruneGraph to use
paginated calls to GetChannelsByOutpoints instead of making one DB call
per outpoint.

sqldb/sqlc/queries/graph.sql

@@ -242,16 +242,6 @@ FROM graph_channels c
 WHERE c.outpoint IN
     (sqlc.slice('outpoints')/*SLICE:outpoints*/);
 
--- name: GetChannelByOutpoint :one
-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 = $1;
-
 -- name: GetChannelAndNodesBySCID :one
 SELECT
     c.*,