autosign / model: Create a table for skipped items

[Spindel]

Ideally we want to use a .limit(100) or similar for the unsigned query, so as to not bite off more than we can chew.

That however won't work unless we have a way to mark an "unwanted" CSR (from the autosigners view) as such. That is not "Rejected", it's simply "not valid for autosigning".

For that we'd need a new table, with only id : csr_id, which we could do a "not in" match on when selecting CSR's to sign.

The reason this comes out is that we can't iterate over all certificates currently, as we run out of RAM.

[Spindel]

https://bitbucket.org/zzzeek/sqlalchemy/wiki/UsageRecipes/WindowedRangeQuery

Won't work on SQLite though