Add method to delete channel state while online

[domZippilli]

Currently, the only way to archive/delete a channel monitor is to delete it while the node is offline, or at least before the node has loaded the monitor into memory and passed references to the ChainMonitor/ChannelManager, etc.

This is because even if you delete the monitor, the node will re-write it on every block connect if it is still running with the monitor in memory.

Please add a way to evict/prune a monitor from memory. Ideally this would also be a good time to validate that the monitor is safe to delete, but from previous discussion I understand this is hard to be certain about, so such a method will certainly put responsibility on the caller to validate conditions such that it's safe to let the channel go.

ref Discord: https://discord.com/channels/915026692102316113/915026887066132481/1204863912051220600