UNITED NATIONS, United States : Russia has lifted its  objections to a UN investigation into chemical attacks in Syria,  clearing the way for the probe to begin, diplomats said Thursday.   The UN Security Council unanimously adopted a resolution August 7  approving a joint investigation by the United Nations and the  Organisation for the Prohibition of Chemical Weapons.   UN Secretary General Ban Ki-moon had followed up by proposing that  the mission be entrusted to three independent experts, and asked the  Security Council for the go ahead to recruit them.   But Russia, which heads the council in September, delayed in responding.   According to diplomats, Moscow wanted guarantees on several points,  notably that the sovereignty of its Syrian ally would be respected and  on the mission's financing.   On Wednesday, Ban addressed a letter to Russian ambassador Vitaly  Churkin giving assurances that the United Nations would "expeditiously  consult" with Damascus on an agree...