chaintnfs/btcdnotify: make cancellation of block epoch synchronous
chaintnfs/btcdnotify: make cancellation of block epoch synchronous

This commit is meant to fix an occasional flake in the interrogation
tests cause by the async nature of the cancellation of block epoch
notifications. This commit modifies the cancellation to now be fully
synchronous which should eliminate this flake.

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