[libcontacts] kick off an UpdateRequest when contacts start to come in

This patch increases the speed of loading contacts by 30% on my device (that is with 10k contacts in the db).

Without it, the seasidecache in the FilterAll phase first adds all of the contacts to m_contactsToAppend before starting to process them in applyPendingContactUpdates.
(actually since postEvent is, obviously, not instantaneous, you should be able to just send it before appending, if it's empty, and drop the bool). but it's late, and i'm tired, so nyah~