channeldb+invoices: revamped external channel+transaction+fee accouting

[Roasbeef]

This is a master tracking issue which follows a larger project that has a goal of plugging the remaining accounting holes within lnd, as well as providing end users with a more coherent interface that they can use to consume complete accounting data delated to lnd.

See this post on the lnd mailing list for a outline of our current design w.r.t the end-user interface of this new sub-system: https://groups.google.com/a/lightning.engineering/forum/#!topic/lnd/bEnRBZvsmB4

Related issues:

• Allow spending unconfirmed push_amount #2579
• [Feature Request] Sending Notifications for fallback_addresses in Invoices #1563
• The closed channels list, should return who initiated the channel-close operation close_initiator #3709

[bitromortac]

Hey, I think #2936 and #3708 are also related. I could try to look a bit deeper into them.