lister.pattern: Restore flushing origin batch in the scheduler

Prior to this commit, the newly introduced check on url validity was consuming the
stream of origins. In effect, this would no longer write origin records regularly.

For all listers, that would translate to flush origins only at the end of the listing
which could take a while for some (e.g. packagist lister has been running for more than
12h currently without writing anything in the scheduler).