introduce SockAddr to store IP addresses

CMakeLists.txt

include/opendht/dht.h

include/opendht/dhtrunner.h

include/opendht/network_engine.h

include/opendht/node.h

include/opendht/node_cache.h

include/opendht/sockaddr.h

include/opendht/utils.h

src/Makefile.am

src/dht.cpp

src/dhtrunner.cpp

src/network_engine.cpp

src/node.cpp

src/node_cache.cpp

src/utils.cpp