lightningd: don't abort on incorrect versions, but try to re-exec.

You still shouldn't do this (you could get some transient failures),
but at least you have a decent chance if you reinstall over a running
daemon, instead of getting confusing internal errors if message
formats have changed.

Changelog-Added: lightningd: we now try to restart if subdaemons are upgraded underneath us.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: #4346

contrib/pyln-testing/pyln/testing/utils.py

@@ -1222,7 +1222,8 @@ class NodeFactory(object):
             'random_hsm',
             'feerates',
             'wait_for_bitcoind_sync',
-            'allow_bad_gossip'
+            'allow_bad_gossip',
+            'start',
         ]
         node_opts = {k: v for k, v in opts.items() if k in node_opt_keys}
         cli_opts = {k: v for k, v in opts.items() if k not in node_opt_keys}