cpg: Enhance downlist selection algorithm

Let's say we have 2 nodes:
- node 2 is paused
- node 1 create membership (one node)
- node 2 is unpaused

Result is that node 1 downlist is selected, so it means that from node 2
point of view, node 1 was never down.

Patch solves situation by adding additional check for largest previous
membership.

So current tests are:
1) largest (previous #nodes - #nodes know to have left)
2) (then) largest previous membership
3) (and last as a tie-breaker) node with smallest nodeid

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Steven Dake <sdake@redhat.com>

exec/cpg.c

@@ -811,16 +811,17 @@ static struct downlist_msg* downlist_master_choose (void)
 		best_members = best->old_members - best->left_nodes;
 		cmp_members = cmp->old_members - cmp->left_nodes;
 
-		if (cmp_members < best_members) {
-			continue;
-		}
-		else if (cmp_members > best_members) {
-			best = cmp;
-		}
-		else if (cmp->sender_nodeid < best->sender_nodeid) {
+		if (cmp_members > best_members) {
 			best = cmp;
+		} else if (cmp_members == best_members) {
+			if (cmp->old_members > best->old_members) {
+				best = cmp;
+			} else if (cmp->old_members == best->old_members) {
+				if (cmp->sender_nodeid < best->sender_nodeid) {
+					best = cmp;
+				}
+			}
 		}
-
 	}
 
 	assert (best != NULL);