dht, dhtrunner: add shutdown stop argument

include/opendht/dht.h

include/opendht/dht_interface.h

include/opendht/dht_proxy_client.h

include/opendht/dhtrunner.h

include/opendht/securedht.h

src/dht.cpp

src/dht_proxy_client.cpp

src/dhtrunner.cpp

tests/dhtproxytester.cpp

tests/dhtproxytester.h