Drop the errorMsg column in the Jobs table

We're not using it anywhere.
Get rid of some obsolete indexes
Use partial indexes

There is no point in indexing rows with common column values like
"finished = 1", since those are the majority of the table. Only the
exceptions ("finished = 0") are interesting. Having smaller tables
should make updates/insertions faster.
hydra-eval-jobs: Use JSON instead of XML

XML::Simple is pretty slow - reading the output for the Nixpkgs jobset
takes half a minute or so. JSON is pretty much instantaneous.
hydra-evaluator: Reduce verbosity
Create Builds with iscurrent set

This should eliminate a subsequent update.
Remove the longDescription field

It's not useful and takes up a lot of space.