solver: check one git coderepo and inbox at a time
With public-inbox-httpd, this mitigates the effect of slow git
blob storage with multiple coderepos configured for an inbox.
It's still synchronous for now (and may need to remain that way
for ->last_check_err), but no longer monopolizes the event loop
when checking multiple coderepos.
We don't yet support multi-inbox scanning, yet; but this also
prepares us for a future where we do.
We'll also support >=40 char blob OIDs in preparation for future
git SHA-256 support, too.