ZeroMQ support for solo mining

Gets new blocks from daemon immediately without polling, saving ~0.5 seconds on average when daemon gets new block from the network. Also saves some CPU cycles because it doesn't need to poll daemon every second.

Testing: add "daemon-zmq-port": 28083 to xmrig's pool config in config.json and run ./monerod --testnet --zmq-pub tcp://127.0.0.1:28083

src/core/config/Config_platform.h

@@ -52,6 +52,7 @@ static const option options[] = {
     { "daemon-poll-interval",  1, nullptr, IConfig::DaemonPollKey         },
     { "self-select",           1, nullptr, IConfig::SelfSelectKey         },
     { "submit-to-origin",      0, nullptr, IConfig::SubmitToOriginKey     },
+    { "daemon-zmq-port",       1, nullptr, IConfig::DaemonZMQPortKey      },
 #   endif
     { "av",                    1, nullptr, IConfig::AVKey                 },
     { "background",            0, nullptr, IConfig::BackgroundKey         },