getmessagelist() is slow for the mapped UID case, so replace some of its
occurences with calls that are optimized for this case, ie
getmessagecount() and uidexists().
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Reviewed-and-tested-by: Vincent Beffara <vbeffara@ens-lyon.fr>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>