[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.
Ah yes, now I see what you mean :) The check in event() does m_contactsToAppend. IsEmpty() so we can use the same test here.