proxy_client: execute callbacks in main thread

include/opendht/dht.h

include/opendht/dht_interface.h

include/opendht/dht_proxy_client.h

include/opendht/dht_proxy_server.h

include/opendht/securedht.h

src/dht_proxy_client.cpp

src/dht_proxy_server.cpp

src/dhtrunner.cpp

src/securedht.cpp