make default panic handler could be customized

[twz915]

What is the purpose of the change

#1074 (comment)

in old version, we could do this

primitive.PanicHandler = func(i interface{}) {
     msentry.CaptureMessage(fmt.Sprintf("%+v", i), nil)
}

but in latest version, default panic handler could not be customized.

when push consume panic, default panic handler should be called (we log and monitor the panic in default panic handler)