[5585] v4 Memfile support complete

[5585] v4 Memfile support complete

Changed subnetID parms to const refs

src/lib/dhcpsrv/memfile_lease_mgr.*
    Added constructors:
    - MemfileLeaseStatsQuery(const SubnetID& subnet_id)
    - MemfileLeaseStatsQuery(const SubnetID& first_subnet_id,
                           const SubnetID& last_subnet_id)

    - MemfileLeaseStatsQuery4(Lease4Storage& storage4, const SubnetID& subnet_id)
    - MemfileLeaseStatsQuery4(Lease4Storage& storage4, const SubnetID& first_subnet_id,
                            const SubnetID& last_subnet_id)

    MemfileLeaseStatsQuery4::start() - altered to set lower/upper iterators
    based on select mode

    Memfile_LeaseMgr::startSubnetLeaseStatsQuery4(const SubnetID& subnet_id)
    Memfile_LeaseMgr::startSubnetRangeLeaseStatsQuery4(const SubnetID& first_subnet_id,
                                                   const SubnetID& last_subnet_id)

src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.cc
    GenericLeaseMgrTest::testLeaseStatsQuery4() - expanded to cover bad values