message: Clear the calculated hash speed at mining start.
  This avoids having the hash speed affected by a gap in mining
  or a change in the number of mining threads, without requiring
  users to call `setminingalgo` on the same algorithm.
diff: @@ -4989,6 +4989,7 @@ UniValue generatecontinuous(const JSONRPCRequest& request)
  result.pushKV("threads", 0);
  result.pushKV("message", "Mining stopped");
  } else {
  + ClearHashSpeed();
  result.pushKV("threads", nThreads);
  if (sWarning.compare(""))
  result.pushKV("message", strprintf("Warning: %s", sWarning.c_str()));
  