BELFAST — Last December, a majority coalition of republican and liberal lawmakers on the city council voted to reduce the number of days the Union Jack flies above the Baroque pediment of City Hall from 365 to 18 — the number of holidays on which British law encourages government buildings to fly the national flag. The vote set off more than three months of protests, rioting and armed clashes in Belfast and other Northern Ireland cities, rekindling the kind of violence many had hoped was extinguished, and exposing tensions that are still simmering.