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

This commit builds upon the prior commit by adding a new method to
allows callers to query the state of all closed (fully) and pending
closed channel within the database.

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