Unsolicited "hold" time + max events parameters

[jadamcrain]

A common, but optional, feature for unsolicited reporting allows the outstation to wait for events to accumulate for a minimum amount of time before transmitting. This feature can reduce network round-trips since each message can carry more events. To ensure that buffers don't overflow, it's useful to also pair with a "max events" parameter that trigger the message even if hold time has not elapsed.

Other implementations may do more complicated things, e.g. having separate parameters for each event class but this seems like overkill. Unsolicited is generally enabled once for the requested classes, it's not like people turn it on and off. When the hold time elapses, or the event minimum is reached, it's required that you send all of the events for the enabled classes, whereas with class polling the master may request the events in separate polls.