The initial TODO was simply to use one ExternalBlock per all batches in the
block. This would require publishing ExternalBlock after the last batch,
requiring knowing the last batch. While we could add such a pipeline, it'd
require:
1) Initial preprocesses using a distinct message from BatchPreprocess
2) An additional message sent after all BatchPreprocess are sent
Unfortunately, both would require tweaks to the SubstrateSigner which aren't
worth the complexity compared to the solution here, at least, not at this time.
While this will cause, if a Tributary is signing a block whose total batch data
exceeds 25 kB, to use multiple transactions which could be optimized out by
'better' local data pipelining, that's an extreme edge case. Given the temporal
nature of each Tributary, it's also an acceptable edge.
This does no longer achieve synchrony over external blocks accordingly. While
signed batches have synchrony, as they embed their block hash, batches being
signed don't have cryptographic synchrony on their contents. This means
validators who are eclipsed may produce invalid shares, as they sign a
different batch. This will be introduced in a follow-up commit.
Due to the ordered message-queue, there's no benefit to multiple emissions as
there's no risk a completion will be missed. If it has yet to be read, sending
another which only be read after isn't helpful.
Simplifies code a decent bit.
