Checker: fix type error

It looks like database.get_check_duration() now returns a
decimal.Decimal instead of a float causing the launch_timeframe
calculation to fail with:

    TypeError: unsupported operand type(s) for -: 'float' and 'decimal.Decimal'

src/ctf_gameserver/checker/master.py

@@ -388,6 +388,7 @@ def update_launch_params(self, tick):
             if check_duration is None:
                 # No complete flag placements so far
                 check_duration = self.tick_duration.total_seconds()
+        check_duration = float(check_duration)
 
         total_tasks = database.get_task_count(self.db_conn, self.service['id'])
         local_tasks = math.ceil(total_tasks / self.checker_count)