message: Add RPC call 'logline' to add lines to debug.log
  This is useful for example to better figure out the timing
  between `debug.log` and the output of a python RPC test script
  itself. By inserting `node.logline(..)` calls into the RPC script,
  common points in time between the different logs can be established.
diff: @@ -119,6 +119,16 @@ UniValue getinfo(const UniValue &params, bool fHelp)
  return obj;
  }
  +UniValue logline(const UniValue &params, bool fHelp)
  +{
  + if (fHelp || params.size() != 1)
  + throw runtime_error("logline 'string'\n"
  + "Writes a string into the log (prefixed with 'rpc-logline: ').\n"
  + "\nResult: None\n");
  + LOGA("rpc-logline: %s\n", params[0].get_str());
  + return NullUniValue;
  +}
  +
  #ifdef ENABLE_WALLET
  class DescribeAddressVisitor : public boost::static_visitor<UniValue>
  {
  @@ -439,6 +449,7 @@ static const CRPCCommand commands[] = {
  {"control", "getinfo", &getinfo, true}, /* uses wallet if enabled */
  {"util", "validateaddress", &validateaddress, true}, /* uses wallet if enabled */
  {"util", "createmultisig", &createmultisig, true}, {"util", "verifymessage", &verifymessage, true},
  + {"util", "logline", &logline, true},
  /* Not shown in help */
  {"hidden", "setmocktime", &setmocktime, true},
  