Expose the set of maturing outpoints from `ChannelMonitor`

[TheBlueMatt]

I haven't dug into this one yet but we should expose the set of maturing outpoints from ChannelMonitors (and ChainMonitor) which will result in a funds claim, including the amount of funds that will appear in that claim. When we have funds tied up in CSVs currently its completely opaque to users, which isn't good.

[TheBlueMatt]

Note that we do not need to expose the set of maturing outpoints which will just result in an HTLC fail/claim, I think, at least not yet.