Moved logic for detecting FQDN changes from the alloc engine to the
kea-dhcp6 server proper.  This allows the change detection to be done after
FQDN generation.

src/bin/dhcp6/dhcp6_srv.*
    Dhcpv6Srv::createNameChangeRequests() - modified to detect when changes
    to a valid lease require a DNS remove

    Dhcpv6Srv::generateFqdn() - updated to store the generated name in
    the current context

src/lib/dhcpsrv/alloc_engine.cc
    AllocEngine::extendLease6()
    AllocEngine::updateLeaseData() - removed logic that checks for FQDN changes

src/bin/dhcp6/dhcp6_messages.*
    Revamped a message