[5487] checkpoint commit

[5487] checkpoint commit

    recount4/6 both work, using queries that return rows for all leases.
    Needs cleanup and optimization

src/lib/dhcpsrv/cql_lease_mgr.cc
    added new class CqlLeaseStatsQuery : public LeaseStatsQuery, CqlExchange

    CqlLeaseMgr::CqlLeaseMgr()
        prepares CqlLeaseStatsQuery statements

    CqlLeaseMgr::startLeaseStatsQuery4()
    CqlLeaseMgr::startLeaseStatsQuery6()
        Create and start CqlLeaseStatsQuery()

src/lib/dhcpsrv/lease_mgr.h
    Added LeaseStatsRows::operator<

src/lib/dhcpsrv/tests/cql_lease_mgr_unittest.cc
    Enabled CqlLeaseMgrTest.recountLeaseStats4/6