Edward McKersie has been warning his clients that times are changing. Employers have ruled the job market for most of the last eight years, but job-seekers are quickly gaining the upper hand, said McKersie, the founder and president of Pro Search, a Portland recruiting and staffing firm “The market’s tight,” McKersie said, and that means it might take longer to fill open positions and require better pay and benefits to attract well-qualified candidates. The changing market is reflected in the state’s plummeting unemployment rate, which has hit lows not seen in more than a decade. The state Department of Labor said Tuesday that the jobless rate in December was 4.0 percent, down 0.1 percentage point from November and the lowest since August 2001.