[master] Merge branch 'trac2320' (DHCPv4 allocation engine)
Conflicts: ChangeLog src/bin/dhcp4/config_parser.cc src/bin/dhcp6/config_parser.cc src/lib/dhcpsrv/lease_mgr.cc src/lib/dhcpsrv/lease_mgr.h src/lib/dhcpsrv/memfile_lease_mgr.cc src/lib/dhcpsrv/subnet.cc src/lib/dhcpsrv/tests/cfgmgr_unittest.cc