lease_checker: add support for empty ranges in get_relative_range/1
lease_checker: add support for empty ranges in get_relative_range/1

jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,jvf/scalaris,scalaris-team/scalaris,scalaris-team/scalaris