peerdiscovery: add multicast node discovery mechanism

CMakeLists.txt

include/opendht/dhtrunner.h

include/opendht/peer_discovery.h

include/opendht/sockaddr.h

src/dhtrunner.cpp

src/peer_discovery.cpp

src/utils.cpp

tests/peerdiscoverytester.cpp

tests/peerdiscoverytester.h