This isuseful to find completely dead channels which are worthy of closing. Changelog-Added: JSON-RPC: `listpeerchannels` field `last_stable_connection` showing when we last held an established channel for a minute or more. Changelog-Added: JSON-RPC: `listclosedchannels` field `last_stable_connection` showing when we last held an established channel for a minute or more. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>