[#4585] D2 Track by FQDN and IP not DHCID

Changed D2's transaction list to a multi_index container.
Transactions are uniquely keyed by FQDN and by address.
New transactions are not started for queued NCRs while
there is an active transaction for either their FQDN
or their address. Requests which exactly duplicate
an ongoing transaction are dequeued and discarded.

modified:   src/bin/d2/d2_update_mgr.cc
modified:   src/bin/d2/d2_update_mgr.h
modified:   src/bin/d2/tests/d2_update_mgr_unittests.cc
modified:   src/lib/d2srv/nc_trans.cc
modified:   src/lib/d2srv/nc_trans.h
modified:   src/lib/dhcp_ddns/ncr_msg.cc
modified:   src/lib/dhcp_ddns/ncr_msg.h