Fix compute next time in green thread of trigger

Time trigger computes next time in the green thread as following codes.

    1 def _trigger_operations(self, expect_run_time, trigger_property):

    2 now = datetime.utcnow()
    3 if expect_run_time > now and (
    4 int(timeutils.delta_seconds(now, expect_run_time)) > 0):
    5 return expect_run_time

             #ending operations to executor

    6 return self._compute_next_run_time(
    7 datetime.utcnow(), trigger_property['end_time'],
    8 trigger_property['format'],
    9 trigger_property['pattern'])

A bug will happen on the following step:
1. 'now' < 'expect_run_time ' and 'expect_run_time' - 'now' < 1(second) on line 3 4
2. the next run time returned on line 6 may still equal to parameter 'expect_run_time'
   if sending operations to executor quickly so that 'datetime.utcnow()' on
   line 7 less than 'expect_run_time'. If this happens, then '_trigger_operations' will
   be invoked immediately.