graph/db+sqldb: use batch fetching during channel&policy migration

Restructue the `migrateChannelsAndPolicies` function so that it does the
validation of migrated channels and policies in batches. So instead of
fetching channel and its policies individually after migrating it, we
wait for a minimum batch size to be reached and then validate a batch of
them together. This lets us make way fewer DB round trips.

sqldb/sqlc/queries/graph.sql

@@ -335,6 +335,59 @@ WHERE
     c.version = @version
   AND c.scid IN (sqlc.slice('scids')/*SLICE:scids*/);
 
+-- name: GetChannelsByIDs :many
+SELECT
+    sqlc.embed(c),
+
+    -- Minimal node data.
+    n1.id AS node1_id,
+    n1.pub_key AS node1_pub_key,
+    n2.id AS node2_id,
+    n2.pub_key AS node2_pub_key,
+
+    -- Policy 1
+    cp1.id AS policy1_id,
+    cp1.node_id AS policy1_node_id,
+    cp1.version AS policy1_version,
+    cp1.timelock AS policy1_timelock,
+    cp1.fee_ppm AS policy1_fee_ppm,
+    cp1.base_fee_msat AS policy1_base_fee_msat,
+    cp1.min_htlc_msat AS policy1_min_htlc_msat,
+    cp1.max_htlc_msat AS policy1_max_htlc_msat,
+    cp1.last_update AS policy1_last_update,
+    cp1.disabled AS policy1_disabled,
+    cp1.inbound_base_fee_msat AS policy1_inbound_base_fee_msat,
+    cp1.inbound_fee_rate_milli_msat AS policy1_inbound_fee_rate_milli_msat,
+    cp1.message_flags AS policy1_message_flags,
+    cp1.channel_flags AS policy1_channel_flags,
+    cp1.signature AS policy1_signature,
+
+    -- Policy 2
+    cp2.id AS policy2_id,
+    cp2.node_id AS policy2_node_id,
+    cp2.version AS policy2_version,
+    cp2.timelock AS policy2_timelock,
+    cp2.fee_ppm AS policy2_fee_ppm,
+    cp2.base_fee_msat AS policy2_base_fee_msat,
+    cp2.min_htlc_msat AS policy2_min_htlc_msat,
+    cp2.max_htlc_msat AS policy2_max_htlc_msat,
+    cp2.last_update AS policy2_last_update,
+    cp2.disabled AS policy2_disabled,
+    cp2.inbound_base_fee_msat AS policy2_inbound_base_fee_msat,
+    cp2.inbound_fee_rate_milli_msat AS policy2_inbound_fee_rate_milli_msat,
+    cp2.message_flags AS policy2_message_flags,
+    cp2.channel_flags AS policy2_channel_flags,
+    cp2.signature AS policy2_signature
+
+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
+    LEFT JOIN graph_channel_policies cp1
+        ON cp1.channel_id = c.id AND cp1.node_id = c.node_id_1 AND cp1.version = c.version
+    LEFT JOIN graph_channel_policies cp2
+        ON cp2.channel_id = c.id AND cp2.node_id = c.node_id_2 AND cp2.version = c.version
+WHERE c.id IN (sqlc.slice('ids')/*SLICE:ids*/);
+
 -- name: GetChannelsByPolicyLastUpdateRange :many
 SELECT
     sqlc.embed(c),