`acknowledge_batch` can only be run if we know what the Batch should be. If we
don't know what the Batch should be, we have to block until we do.
Specifically, we need the block number associated with the Batch.
Instead of blocking over the Scanner API, the Scanner API now solely queues
actions. A new task intakes those actions once we can. This ensures we can
intake the entire Substrate chain, even if our daemon for the external network
is stalled at its genesis block.
All of this for the block number alone seems ridiculous. To go from the block
hash in the Batch to the block number without this task, we'd at least need the
index task to be up to date (still requiring blocking or an API returning
ephemeral errors).
Has fetched blocks checked to be the indexed blocks. Has scanned outputs be
sorted, meaning they aren't subject to implicit order/may be non-deterministic
(such as if handled by a threadpool).
Scan now only handles External outputs, with an associated essay going over
why. Scan directly creates the InInstruction (prior planned to be done in
Report), and Eventuality is declared to end up yielding the outputs.
That will require making the Eventuality flow two-stage. One stage to evaluate
existing Eventualities and yield outputs, and one stage to incorporate new
Eventualities before advancing the scan window.
