[#805,!6-p] CSVLeaseFile4 ensures either hwaddr or client id for non-declined leases

[#805,!6-p] CSVLeaseFile4 ensures either hwaddr or client id for non-declined leases

Declined leases are expected to have neither hardware address nor client
id.  All others have must have at least one of them.

src/lib/dhcpsrv/csv_lease_file4.cc
    CSVLeaseFile4::append() - throws if a lease has no hardware address,
    no client id and is not in STATE_DECLINED

    CSVLeaseFile4::next(Lease4Ptr& lease) - discards rows if they have
    neither hardware addr nor client id and are not in STATE_DECLINED

src/lib/dhcpsrv/tests/csv_lease_file4_unittest.cc
src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.cc
src/lib/dhcpsrv/tests/lease_file_loader_unittest.cc
src/bin/lfc/tests/lfc_controller_unittests.cc
    Updated tests.