By Pavel Polityuk and Conor Humphries KYIV/SLOVYANSK, Ukraine (Reuters) -Russian forces waged an all-out assault on Tuesday to encircle Ukrainian troops in twin cities straddling a river in eastern Ukraine, a battle which could determine the success or failure of Moscow’s main campaign in the east. Exactly three months after President Vladimir Putin ordered troops into Ukraine, authorities in its second-largest city Kharkiv re-opened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment. The move reflected Ukraine’s biggest military success i…