[#805,!6-p] Addressed review comments

[#805,!6-p] Addressed review comments

src/lib/dhcpsrv/csv_lease_file4.*
src/lib/dhcpsrv/tests/csv_lease_file4_unittest.cc
    - minor cleanup

src/lib/dhcpsrv/csv_lease_file6.*
    CSVLeaseFile6::append() - now throws if DUID is empty and
    state is not STATE_DECLINED

src/lib/dhcpsrv/tests/csv_lease_file6_unittest.cc
    Updated tests to verify duid/state logic