channeldb: use a read-txn rather than a write-txn in FetchChannelEdgesByID
channeldb: use a read-txn rather than a write-txn in FetchChannelEdgesByID

This commit modifies the FetchChannelEdgesByID slightly to use a
read-only transaction rather than a write-only transaction. As a result
we’ll no longer extraneously consume a writer’s slot when we’re only
reading data from the database.

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