public long subscribeInSync(final InSyncListener listener) {
    return subscribeInSync(listener, Synchronizer.DEFAULT_IN_SYNC_TOLERANCE);
  }
Add a listener that will be notified when this node's sync status changes. A node is considered
in-sync if the local chain height is no more than {@code SYNC_TOLERANCE} behind the highest
estimated remote chain height.

@param listener The callback to invoke when the sync status changes
@return An {@code Unsubscriber} that can be used to stop listening for these events
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/state/SyncState.java