contractcourt: ensure we always cancel block epoch notifications after use

In this commit, we fix an existing grouting leak within the contract
court package. If a goroutine dies, but it doesn’t actually cancel the
block epoch notification that it requested, then it’s possible to leak
thousands of gorutines. To remedy this situation, we ensure that we’ll
*always* cancel the epoch notification once the goroutine has exited.

contractcourt/chain_arbitrator.go

@@ -395,6 +395,7 @@ func (c *ChainArbitrator) Start() error {
 			c.chanSource.DB, arbCfg, c.cfg.ChainHash, chanPoint,
 		)
 		if err != nil {
+			blockEpoch.Cancel()
 			return err
 		}
 		arbCfg.MarkChannelResolved = func() error {

contractcourt/contract_resolvers.go

@@ -865,6 +865,8 @@ func (h *htlcOutgoingContestResolver) Resolve() (ContractResolver, error) {
 	if err != nil {
 		return nil, err
 	}
+	defer blockEpochs.Cancel()
+
 	for {
 		select {
 
@@ -1058,7 +1060,10 @@ func (h *htlcIncomingContestResolver) Resolve() (ContractResolver, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer preimageSubscription.CancelSubcription()
+	defer func() {
+		preimageSubscription.CancelSubcription()
+		blockEpochs.Cancel()
+	}()
 	for {
 
 		select {