[5007] Addressed review comments

[5007] Addressed review comments

This is solution #2:

src/lib/dhcpsrv/alloc_engine.h
src/lib/dhcpsrv/alloc_engine.cc
    AllocEngine::extendLease6() - replaced logic to set the context flags with
    simply adding the original lease to the changed_leases_ list.

    AllocEngine::updateLeaseData() - removed logic to set the context flags.

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::createNameChangeRequests() - replaced the context flag check
    with logic which looks for candidate IA addresses in the ctx.changed_leases_
    list.  If found and the FDQN doman name has not changed, we move on to the
    next candidate.

src/bin/dhcp6/tests/fqdn_unittest.cc
    TEST_F(FqdnDhcpv6SrvTest, createNameChangeRequestsNoAddr) - removed
    testing of context flag permutations