[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.
    bool sendKick = false;
    if (m_contactsToAppend.isEmpty())
        sendKick = true;
    m_contactsToAppend.insert(....);
    if (sendKick)
        QCoreApplication::postEvent(....);

was my understanding of matt's comment. send the event conditionally, not unconditionally.