Instead of issuing a query for every variant, we assemble a set of
variant ids and query `WHERE variant_id in (1,...)'. This has not only
the effect of higher optimization potential for the database, but also
the query is issued before any result can come back. This will avoid an
overfull receive queue within the job server.
Change-Id: I5b1c60f92b97741ce26d9e50760b601929cef44f
61050b3760