BUG/MEDIUM: connections: Make sure we remove CO_FL_SESS_IDLE on disown.

When for some reason the session is not the owner of the connection anymore,
make sure we remove CO_FL_SESS_IDLE, even if we're about to call
conn->mux->destroy(), as the destroy may not destroy the connection
immediately if it's still in use.
This should be backported to 1.9.
u
diff --git a/src/backend.c b/src/backend.c
index d7695bf..e41689d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1406,6 +1406,7 @@
 				session_unown_conn(s->sess, old_conn);
 				old_conn->owner = sess;
 				if (!session_add_conn(sess, old_conn, old_conn->target)) {
+					old_conn->flags &= ~CO_FL_SESS_IDLE;
 					old_conn->owner = NULL;
 					old_conn->mux->destroy(old_conn->ctx);
 				} else