Parameterize event buffer based on maximum numbers of events for each class

[jadamcrain]

Currently, the event buffer is configured based on the maximum number of events for each type. This was how OpenDNP3 did thing, but I don't think it's he best way:

1. It's easy to misconfigure and it makes it difficult to add support for new event types without breaking configuration
2. DNP3 event reporting is really geared around classes, not individual types.
3. You already have to tracking the number of each class in the event buffers to support the related IIN bits.

Using a reserved # for each class is a better mechanism:

1. It's simpler and more natural to configure things this way since each individual point is assigned to a class.
2. It's more efficient: it reduces the likelihood of buffer overflow by allowing various types to share space, e.g. Analog and Counters could share all reserved the class 2 space.
3. It allows new types to be added without breaking configuration since there will only ever be 3 classes.