channeldb: add new method to db, MarkChanFullyClosed
channeldb: add new method to db, MarkChanFullyClosed

This commit adds a new method to the database which allows callers to
mark a channel as fully closed once certain conditions have been
reached. If a channel was cooperatively closed, then it can be marked
as fully closed as soon as the channel has been confirmed. If the
channel was marked as force closed, then it should be marked as closed
as soon as _all_ the funds in limbo have been swept.

LightningNetwork/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,LightningNetwork/lnd