wtclient: decrement pending tasks stats upon accepted task

We'd never decrement the number of pending backups upon a watchtower
accepting one, making it confusing for users to determine whether their
backups have actually been accepted. Along the way, we also rename
NumTasksReceived to NumTasksPending to better reflect its purpose.

lnrpc/wtclientrpc/wtclient.go

@@ -336,7 +336,7 @@ func (c *WatchtowerClient) Stats(ctx context.Context,
 
 		stats.NumTasksAccepted += stat.NumTasksAccepted
 		stats.NumTasksIneligible += stat.NumTasksIneligible
-		stats.NumTasksReceived += stat.NumTasksReceived
+		stats.NumTasksPending += stat.NumTasksPending
 		stats.NumSessionsAcquired += stat.NumSessionsAcquired
 		stats.NumSessionsExhausted += stat.NumSessionsExhausted
 	}
@@ -344,7 +344,7 @@ func (c *WatchtowerClient) Stats(ctx context.Context,
 	return &StatsResponse{
 		NumBackups:           uint32(stats.NumTasksAccepted),
 		NumFailedBackups:     uint32(stats.NumTasksIneligible),
-		NumPendingBackups:    uint32(stats.NumTasksReceived),
+		NumPendingBackups:    uint32(stats.NumTasksPending),
 		NumSessionsAcquired:  uint32(stats.NumSessionsAcquired),
 		NumSessionsExhausted: uint32(stats.NumSessionsExhausted),
 	}, nil