public void writeSnapshots(String hostname, List<Snapshot> snapshots, NestedTransaction transaction) {
        Path path = snapshotsPath.append(hostname);
        Slime serializedSnapshots = SnapshotSerializer.toSlime(snapshots);
        CuratorTransaction curatorTransaction = db.newCuratorTransactionIn(transaction);
        curatorTransaction.add(createOrSet(path, Exceptions.uncheck(() -> SlimeUtils.toJsonBytes(serializedSnapshots))));
    }
Returns the instant after which a firmware check is required, if any.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/CuratorDb.java