[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.
Yes, what Robin said.  If the empty lists remove themselves from the map then the test is just:
    if (m_contactsToAppend.isEmpty()) { postEvent(...); }
    m_contactsToAppend.insert(...);
right?