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.

sqldb/sqlc/querier.go

@@ -96,6 +96,12 @@ type Querier interface {
 	// and version columns).
 	InsertChannelMig(ctx context.Context, arg InsertChannelMigParams) (int64, error)
 	InsertClosedChannel(ctx context.Context, scid []byte) error
+	// NOTE: This query is only meant to be used by the graph SQL migration since
+	// for that migration, in order to be retry-safe, we don't want to error out if
+	// we re-insert the same policy (which would error if the normal
+	// UpsertEdgePolicy query is used because of the constraint in that query that
+	// requires a policy update to have a newer last_update than the existing one).
+	InsertEdgePolicyMig(ctx context.Context, arg InsertEdgePolicyMigParams) (int64, error)
 	InsertInvoice(ctx context.Context, arg InsertInvoiceParams) (int64, error)
 	InsertInvoiceFeature(ctx context.Context, arg InsertInvoiceFeatureParams) error
 	InsertInvoiceHTLC(ctx context.Context, arg InsertInvoiceHTLCParams) (int64, error)