2.4.0

• OpenDHT now requires C++17
• build/cmake: removed OPENDHT_LTO, use CMAKE_INTERPROCEDURAL_OPTIMIZATION instead
• infohash: allow building from std::string_view
• value: allow building queries from std::string_view
• c: dht_runner_run returns a standard error code in case of failure
• c: new bindings dht_runner_is_running, dht_runner_get_bound_port, dht_privatekey_decrypt
• c: dht_runner_ping can be provided a callback argument
• ci: updated Dockerfiles
• ci: removed travis