Fixed typos and added decline lease checks for DHCPv6

src/lib/dhcp/duid.h
src/lib/dhcp/duid.cc
src/lib/dhcp/tests/duid_unittest.cc
    - Replaced DUID::generateEmpty() with DUID::EMPTY() which returns
    a constant reference to a static DUID instance.  This facilitates
    comparisions.
    - Updated TEST(DuidTest,empty) accordingly

src/lib/dhcpsrv/csv_lease_file6.cc
    CSVLeaseFile6::next() - added test to permit the Empty DUID
    only if state is STATE_DECLINED.

src/lib/dhcpsrv/lease.cc
    Lease6::decline() - modified to use DUID::EMPTY().

src/lib/dhcpsrv/tests/csv_lease_file4_unittest.cc
    Fixed Typos

src/lib/dhcpsrv/tests/csv_lease_file6_unittest.cc
    TEST_F(CSVLeaseFile6Test, declinedLeaseTest) - new test to
    verify proper handling of declined leases