Optimize DataStore::setItemsFlags to not fetch item flags again.

Attaching perf to akonadiserver while deleting 10k items
(and possibly some background mailcheck...) showed this in Hotspot:
http://www.davidfaure.fr/2020/hotspot_pimitem_flags.png

Clearly this is calling PimItem::flags() (which makes a SQL query)
twice, once in ItemCreateHandler and again in DataStore::setItemFlags.
Just pass the flags from the former to the latter.