Add consensus filter to AddRefetchNeededToBlock migration

It is unnecessary to set `refetch_needed` to blocks that do not yet have a consensus, so this small change can save some time in the migration and avoid some checking from the temporary fetcher.

apps/explorer/priv/repo/migrations/20190513134025_add_refetch_needed_to_block.exs

@@ -6,6 +6,6 @@ defmodule Explorer.Repo.Migrations.AddRefetchNeededToBlock do
       add(:refetch_needed, :boolean, default: false)
     end
 
-    execute("UPDATE blocks SET refetch_needed = TRUE;", "")
+    execute("UPDATE blocks SET refetch_needed = TRUE WHERE consensus", "")
   end
 end