refactored Existing_Conversations_Model::update_last_update_check() to handle multiple conversations
refactored Existing_Conversations_Model::is_up_to_date to return an array of conversation_id's that need to be updated
Existing_Conversations_Controller::get() setup to handle multiple conversations
Refactored Existing_Conversations_Model::update_last_update_check() to take a simple list of conversation id's