GLOBAL MARKETS-World shares at 6-month low as growth worries mount

* MSCI All-Country World index hits 6-month low* Benchmark on track for third straight weekly loss* Investors fret about global slowdown as Fed stimulus endsBy Francesco CanepaLONDON, Oct 10 (Reuters) - Shares across the world fell sharply on Friday, pushing a global index to a six-month low, as investors worried about the prospect of a widespread economic slowdown while U.S. monetary stimulus nea