Add indexes for `thr_parent` column in `item`

This should avoid 100% system load on similar queries

select channel_portable_id as hash from channel left join item on channel.channel_id = item.uid where ( item.thr_parent = 'https://hz.raspberryip.com/item/64974877-e5ea-4482-bbfa-5aebb77923d5' OR item.parent_mid = 'https://hz.raspberryip.com/item/64974877-e5ea-4482-bbfa-5aebb77923d5' );