MILAN (Reuters) -Sport streaming app DAZN will introduce a premium subscription and tighten rules on access to its services in Italy as it strives to boost revenue after a multi-billion euro deal to show top soccer games in the country. Backed by billionaire Len Blavatnik, DAZN last year won the right to screen Serie A live matches in Italy for three seasons with a 2.5 billion euro ($2.7 billion) bid, in one of the largest deals for the video-streaming service in Europe. But the contract has proved challenging for DAZN, with the service experiencing technical outages at the start of last seaso…