[master] Merge branch 'trac3985' (automated declined v6 lease recovery)

diff --cc doc/guide/dhcp6-srv.xml
Simple merge

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

diff --cc src/lib/dhcpsrv/alloc_engine.cc
Simple merge

diff --cc src/lib/dhcpsrv/alloc_engine_messages.mes
Simple merge

diff --cc src/lib/dhcpsrv/tests/alloc_engine_expiration_unittest.cc
Simple merge

diff --cc src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.cc
Simple merge

diff --cc src/lib/dhcpsrv/tests/memfile_lease_mgr_unittest.cc
Simple merge