Add new index to speed up collection of transactions by type, subtype

[frankTheTank72]

We should add a new index to the DB:

ALTER TABLE transaction ADD INDEX transaction_type_subtype_idx (type, subtype`) USING BTREE;

This gives a massive speed up from around 30sec to 0.14 sec for this query :

SELECT id FROM transaction WHERE (transaction.subtype = 8 AND transaction.type = 2) ORDER BY transaction.height DESC;