BRUSSELS — European Union leaders, faced with a staggering migration crisis and deep divisions over how to tackle it, managed to agree early Thursday to boost border controls to ease the influx and to send $1.1 billion to international agencies helping refugees at camps near their home countries. The leaders also said that task forces of European experts sent to help register and screen migrants in so-called hotspots must be fully operational in Greece and Italy, and perhaps also Bulgaria, by November.