dispute-coordinator sometimes fails importing dispute votes on startup due to missing CandidateReceipt

[tdimitrov]

While testing disputes on versi I stumbled upon the following error:

Cannot import votes, without `CandidateReceipt` available! session=703 ...

After some debugging I discovered that:

• The failure happens only when onchain votes are imported. This is logical as it is the only possible case when dispute-coordinator relies on CandidateReceipt being available in the DB.
• The reason for the failure is that the vote is scraped before (1) it is imported by dispute-distribution and (2) the backing vote is scraped onchain.

The reason for (1) is that dispute-coordinator was still waiting for a leaf and ignored the import from dispute-distribution. This is confirmed by:

TRACE tokio-runtime-worker parachain::dispute-distribution: No batch yet - triggering immediate import candidate_hash=.....
DEBUG tokio-runtime-worker parachain::dispute-distribution: error=ImportCanceled(.....)

The reason for (2) is that backing votes (which provide the CandidateReceipt) are not scraped beyond finality (as pointed by @eskimor).

The remedy is to adjust the oinchain scraper not to prune 10 blocks into finality, but also scrape ten blocks beyond it. This will not fully resolve the issue but will make it almost negligible.