Solo mining: added job timeout (default is 15 seconds)

It's important to update jobs frequently to get new transactions into the block template. See https://rucknium.me/posts/monero-pool-transaction-delay/ for more details.

src/base/net/stratum/DaemonClient.h

@@ -104,6 +104,7 @@ private:
     String m_blocktemplateStr;
     String m_currentJobId;
     String m_prevHash;
+    uint64_t m_jobSteadyMs = 0;
     String m_tlsFingerprint;
     String m_tlsVersion;
     Timer *m_timer;