Batch building should use arcs

[Mirko-von-Leipzig]

The batch builder in the block-producer rework should take in Arc<ProvenTransaction> to prevent cloning them.

#530 (comment)

[Mirko-von-Leipzig]

I agree. I don't think there is a good reason why TransactionBatch::new() cannot take something like IntoIterator<Item = AsRef<ProvenTransaction> as input.

Originally posted by @bobbinth in #541 (comment)

[Mirko-von-Leipzig]

It appears I managed to rebase out these changes somehow. Will need to re-do #544 once #562 lands on next.

[Mirko-von-Leipzig]

Closed by #570