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/querier.go

@@ -42,6 +42,7 @@ type Querier interface {
 	GetChannelFeaturesBatch(ctx context.Context, chanIds []int64) ([]GraphChannelFeature, error)
 	GetChannelPolicyByChannelAndNode(ctx context.Context, arg GetChannelPolicyByChannelAndNodeParams) (GraphChannelPolicy, error)
 	GetChannelPolicyExtraTypesBatch(ctx context.Context, policyIds []int64) ([]GetChannelPolicyExtraTypesBatchRow, error)
+	GetChannelsByIDs(ctx context.Context, ids []int64) ([]GetChannelsByIDsRow, error)
 	GetChannelsByOutpoints(ctx context.Context, outpoints []string) ([]GetChannelsByOutpointsRow, error)
 	GetChannelsByPolicyLastUpdateRange(ctx context.Context, arg GetChannelsByPolicyLastUpdateRangeParams) ([]GetChannelsByPolicyLastUpdateRangeRow, error)
 	GetChannelsBySCIDRange(ctx context.Context, arg GetChannelsBySCIDRangeParams) ([]GetChannelsBySCIDRangeRow, error)