optimize ERC721 inventory query

Inventory tab for ERC721 doesn't load
any token transfers because query is timing
out.

Looking into the query, it does a couple of
unnessary joins:
1. with `transactions` for `block_number`. But
token_transfers already have block_number
2. with `tokens` to check if `token_transfers` are
for ERC-721 token. But the query is only executed only
for ERC 721 tokens

One more issue that I found is that the query uses distinct
by `token_id`. But we don't have any indexes on `token_id`.
So this PR adds index on `token_id`.

apps/explorer/lib/explorer/chain/token_transfer.ex

@@ -238,12 +238,8 @@ defmodule Explorer.Chain.TokenTransfer do
   def address_to_unique_tokens(contract_address_hash) do
     from(
       tt in TokenTransfer,
-      join: t in Token,
-      on: tt.token_contract_address_hash == t.contract_address_hash,
-      join: ts in Transaction,
-      on: tt.transaction_hash == ts.hash,
-      where: t.contract_address_hash == ^contract_address_hash and t.type == "ERC-721",
-      order_by: [desc: ts.block_number],
+      where: tt.token_contract_address_hash == ^contract_address_hash,
+      order_by: [desc: tt.block_number],
       distinct: tt.token_id,
       preload: [:to_address],
       select: tt

apps/explorer/priv/repo/migrations/20190827120224_add_index_on_token_transfer_token_id.exs

@@ -0,0 +1,7 @@
+defmodule Explorer.Repo.Migrations.AddIndexOnTokenTransferTokenId do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists(index(:token_transfers, [:token_id]))
+  end
+end