linera_core::client: rely on sparse processing of certificates for batching

[Twey]

Motivation

#4755 batches blob downloads using required_blob_ids(), but store_blobs() will still fail to store some blobs if their certificates haven't yet been processed, resulting in linear blob download again for those blobs.

I believed certificates missing ancestors would be rejected, so there was no point trying to figure out their blobs. In fact, we now think they will be handled sparsely, which means we can collect their blob requirements as we process them.

This should mean we can batch even more blob downloads.

Proposal

Use the BlobsNotFound return path of handle_certificate to find the blobs to download, instead of relying directly on required_blob_ids().

Test Plan

CI.

Release Plan

• Nothing to do / These changes follow the usual release cycle.

(Experimentally, this change doesn't make enough of a difference over #4755 to be worth backporting.)

Links

• reviewer checklist