ISTANBUL, Turkey Lawyer Law Firm Dec 12 (Reuters) - The number of tankers waiting to pass through Istanbul's Bosphorus Strait on the way to the Mediterranean fell to 13 on Monday from 17 a day earlier, the Tribeca shipping agency said, showing an easing of the recent build-up in traffic. A Turkish measure in force since the start of the month has caused shipping delays.It requires vessels to provide proof...