proxyserver: add lock_guard to avoid to manually unlock

include/opendht/callbacks.h

include/opendht/dht.h

include/opendht/dhtrunner.h

include/opendht/securedht.h

src/dht_proxy_client.cpp

src/dht_proxy_server.cpp

src/dhtrunner.cpp

src/securedht.cpp

tools/dhtnode.cpp

tools/tools_common.h