fix more sms sync issues and try speed up the sync

1. The initial sync is just too slow that causes the desktop SMS app to fail when sending and replying to message threads because the first message is never available in time for the conversation id to populate, or https://invent.kde.org/network/kdeconnect-kde/-/blob/master/plugins/sms/conversationsdbusinterface.cpp#L193 this fails when sending a SMS from the SMS app since conversationID is missing. Using a threaded environment seems to really speeds up the initial first messages sync.

2. Sync when receiving and sending SMS fails in conversations because in sendLatestMessage() the latest timestamp is always reported as 0, so the desktop SMS app never receives any updates when new messages are sent / received, adds a method to resolve the latest message timestamp whenever the ContentObserver receives a change in status of new incoming and outgoing SMS.