code: function getLastCoinbaseUnitRoundIndex(conn, address, cb){
      if (!conn)
          return getLastCoinbaseUnitRoundIndex(db, address, cb);
      if(!validationUtils.isNonemptyString(address))
          return cb("param address is null or empty string");
      if(!validationUtils.isValidAddress(address))
          return cb("param address is not a valid address");
          conn.query(
          "SELECT round_index FROM units JOIN unit_authors USING(unit)  \n\
          WHERE is_stable=1 AND sequence='good' AND pow_type=? \n\
           AND address=? ORDER BY round_index DESC LIMIT 1", 
           [constants.POW_TYPE_COIN_BASE, address],
          function(rows){
              if(rows.length === 0)
                  return cb(null, 0);
              cb(null, rows[0].round_index);
          }
      );
  }
docs: Get the round index of address's last coinbase unit.
  
  @param	{obj}	    conn      if conn is null, use db query, otherwise use conn.
  @param	{string}	address
  @param	{function}	cb( err, roundIndex ) callback function
  If there's error, err is the error message and roundIndex is null.
  If the address hasn't launch coinbase unit, roundIndex is 0.
  If there's no error, roundIndex is the result.