totemsrp: Improve logging of left/down nodes

This patch from Hideo Yamauchi improves the logging of
whether nodes leave the cluster cleanly or uncleanly,
making it easier to determine if a node ws shut down
by the operator. There is also the possibility that a
LEAVE message could get missed (due to the node being
in flush state) so this can also make that clearer.

The modifications are as follows.

Change 1) I added the list which maintained LEAVE node to totemsrp.
Change 2) I added registration, a search, the handling of to clear LEAVE
node.
Change 3) I added the output to log.
Change 4) I changed an output level of the log.

Signed-off-by: Hideo Yamauchi <renayama19661014@ybb.ne.jp>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>