KeyregTxn should not be instantiable.

[jkschin]

Problem

In #209, we created the classes KeyregOnlineTxn, KeyregOfflineTxn and KeyregNonparticipatingTxn. They are all subclassed from KeyregTxn. Prior to #209, KeyregTxn can directly be instantiated and the differentiation is through what attributes are in that object.

Solution

I propose that we deprecate KeyregTxn since I don't think there's a use case for calling it directly.

Dependencies

Nil.

Urgency

This isn't urgent but it keeps the code base clean. If we definitely don't want KeyregTxn to be used in future, then it should be deprecated.

[jkschin]

@jasonpaulos I created this as a follow up to what we discussed in #209, I won't be working on it now and it's for posterity. Would love to hear your thoughts on it.