在vnpy/trader/gateway.py中的BaseGateway.on_tick()中有这样的语句：
def on_tick(self, tick: TickData) -> None:
"""
Tick event push.
Tick event of a specific vt_symbol is also pushed.
"""
self.on_event(EVENT_TICK, tick)
self.on_event(EVENT_TICK + tick.vt_symbol, tick)
其中EVENT_TICK + tick.vt_symbol这个事件会在哪个里面用到和处理呢？
