We do not need to be able to calculate multiple SignatureHash versions for a
single transaction format; instead, we use the transaction format to determine
the SigVersion.
The consensus branch ID *does* need to be passed in from the outside, as only
the caller knows the context in which the SignatureHash is being calculated
(ie. mempool acceptance vs. block validation).
JoinSplit signature verification has been moved into ContextualCheckTransaction,
where the consensus branch ID can be obtained.
The argument to the sign command for zcash-tx has been modified to take a height
in addition to the optional sigtype flags.
