optimize token token transfers query

the longes part of the query is ordering.
Removing order clause from the query makes a
query much faster. It's because by default indexes
are ordered in ASC order

apps/explorer/priv/repo/migrations/20191009121635_add_token_transfer_sorting_indexes.exs

@@ -0,0 +1,12 @@
+defmodule Explorer.Repo.Migrations.AddTokenTransferSortingIndexes do
+  use Ecto.Migration
+
+  def change do
+    create(
+      index(
+        :token_transfers,
+        ["block_number DESC", "log_index DESC"]
+      )
+    )
+  end
+end