BRUSSELS — Struck by suicide bombers 10 days ago, the airport known as “the heart of Europe” has stopped beating temporarily, causing headaches for thousands of business and vacation travelers, and painful financial losses for Belgium’s capital and tourism industry. It’s still unclear when Brussels Airport will reopen. But a key hurdle appeared to have been cleared Friday evening when police unions, which had threatened a strike, reached a preliminary deal with government negotiators on how to bolster security to minimize more violence.