add indexes for token instances fetching queries

Without these indexes the query can not be finished.

But on staging requests for `tokenURI` are timing out.
That's why we have so many records with empty metadata.

apps/explorer/priv/repo/migrations/20191007082500_add_indexes_for_token_instances_query.exs

@@ -0,0 +1,8 @@
+defmodule Explorer.Repo.Migrations.AddIndexesForTokenInstrancesQuery do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists(index(:token_instances, [:token_id]))
+    create_if_not_exists(index(:tokens, [:type]))
+  end
+end