config+server: expose HistoricalSyncInterval flag

This allows us to set how often the daemon should routinely perform
historical graph syncs.

server.go

@@ -689,6 +689,7 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB, cc *chainControl,
 		RotateTicker:              ticker.New(discovery.DefaultSyncerRotationInterval),
 		HistoricalSyncTicker:      ticker.New(discovery.DefaultHistoricalSyncInterval),
 		ActiveSyncerTimeoutTicker: ticker.New(discovery.DefaultActiveSyncerTimeout),
+		HistoricalSyncTicker:      ticker.New(cfg.HistoricalSyncInterval),
 		NumActiveSyncers:          cfg.NumGraphSyncPeers,
 	},
 		s.identityPriv.PubKey(),