Kill builds that produce more than 64 MiB of log output
hydra-build: Don't send a giant query to the database

We had Postgres barfing with this error:

  DBIx::Class::Storage::DBI::_dbh_execute(): DBI Exception: DBD::Pg::st execute failed: ERROR: stack depth limit exceeded

because the ‘drvpath => [ @dependentDrvs ]’ in failDependents can
cause a query of unbounded size.  (In this specific case there was a
failure of Bison, which has > 10000 dependent derivations.)  So now we
just get all scheduled builds from the DB.