[master] Merge branch 'trac3555' (MAC storage in Lease6, memfile)

diff --cc src/bin/dhcp4/dhcp4_srv.cc
Simple merge

diff --cc src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
Simple merge

diff --cc src/bin/dhcp6/dhcp6_srv.cc
Simple merge

diff --cc src/bin/dhcp6/tests/hooks_unittest.cc
Simple merge