Add channel_id to SpendableOutputs event

[tlulu]

What

There currently isn't a way to link SpendableOutputs and ChannelMonitor. One can get around this by checking a ChannelMonitor's claimable balances

ldkService.getChannelMonitors().filter { it._claimable_balances.isEmpty() }

Why

The channel_id should be added to SpendableOutputs event which could be persisted so clients of LDK can:

1. easily prune its corresponding ChannelMonitor after processing this event
2. confirm with a database query if a ChannelMonitor is safe to prune
3. reconcile funds using database queries

[jbesraa]

picking this up please!