Prune `ChannelMonitor`s in `background-processor`

[ariard]

Problem is ChannelMonitor become out-of-date due to different reasons (reorg, revocations, close, ...). We should get them out of SimpleManyChannelMonitor when it's accurate.

I'm afraid that we need to keep all previous state until channel close, I mean that's a good component of LN security model..

[TheBlueMatt]

We now have logic to detect when a ChannelMonitor is prunable, so we literally just need to call it from the BP.