[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.
Hmm it's more complicated than that. event() will queue a new event after it has called applyPendingContactUpdates, which might have just emptied the list. So in the worst case, with UpdateRequest and contactsAvailable alternating, every cycle would add two requests and only consume one.

I'll think about a tidy solution for this.