Adds persist_network_graph to Channel Manager Event Handler

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[ConorOkus]

Is it worth adding the default Scorer to chain_sync_completed or at least explicitly showing it can be null? - https://lightningdevkit.org/tutorials/build_a_node_in_java/#12-sync-channelmonitors-and-channelmanager-to-chain-tip

[arik-so]

Adding the scorer is necessary to obtain a payer object that can handle invoices, so it's probably worth an example.