takeover: prevent crash by avoiding free in traverse on RST timeout

takeover: prevent crash by avoiding free in traverse on RST timeout

After 5 attempts to send a RST to a client without any response, we free
"con"; this is done during a traverse.  This frees the node we are walking
through (the node is made a child of "con" down in rb_tree.c's
trbt_create_node() (Valgrind would catch this, as Martin confirmed).

So, we create a temporary parent and reparent onto that; then we free
that parent after the traverse, thus deleting the unwanted nodes.

CQ:S1019041
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>