channeldb: introduce FetchOpenChannels
channeldb: introduce FetchOpenChannels

This commit introduces/re-writes a method FetchOpenChannels for
channeldb, which returns all currently active/open channels for a
particular peerID.

ddustin/lnd,LightningNetwork/lnd,BitfuryLightning/lnd,BitfuryLightning/lnd,LightningNetwork/lnd,aakselrod/lnd,bryanvu/lnd,bryanvu/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd