BATROVCI, Serbia/OPATOVAC, Croatia (Reuters) - Serbia threatened to retaliate on Tuesday after Croatia halted all cargo traffic from its eastern neighbor, with relations between the ex-Yugoslav republics starting to fray over the flow of migrants across their joint border. More than 30,000 migrants, many of them Syrian refugees, have entered European Union member Croatia from Serbia since Tuesday last week, when Hungary barred their entry to the EU over its southern border with Serbia with a metal fence. They are being bussed by Serbia direct to the Croatian border, having entered Serbia from Macedonia. Croatia says it cannot cope with...