conversationmodel: update lastMessageUid when removing last interaction

Actually, conversation.lastMessageUid still has the removed last
interaction uid. If this interaction is removed, this attribute must
be updated.

+ Add a unit test

Change-Id: Icfce0a54d7a88fdf71469127392460048f491ba8
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>