add network id to distinguish different networks

include/opendht/callbacks.h

include/opendht/dhtrunner.h

include/opendht/network_engine.h

include/opendht/routing_table.h

include/opendht/utils.h

src/dht.cpp

src/network_engine.cpp

tools/dhtchat.cpp

tools/dhtnode.cpp

tools/dhtscanner.cpp

tools/tools_common.h