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/base/kernel/interfaces/IConfig.h

@@ -85,6 +85,7 @@ public:
         DnsIPv6Key           = 1053,
         DnsTtlKey            = 1054,
         SpendSecretKey       = 1055,
+        DaemonZMQPortKey     = 1056,
 
         // xmrig common
         CPUPriorityKey       = 1021,