lightningd: generically fail requests when plugin dies.

We had special code to fail a forwarded request, but not for an
internally-generated request.  Instead, we should pretend the (dead)
plugin responded with a PLUGIN_TERMINATED error, and handle the
request through the normal paths.

This breaks the case where a plugin crashes (or stops itself) in a
hook, so we handle that next.

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

lightningd/plugin.h

@@ -76,10 +76,6 @@ struct plugin {
 	/* Currently pending requests by their request ID */
 	STRMAP(struct jsonrpc_request *) pending_requests;
 
-	/* An array of currently pending RPC method calls, to be killed if the
-	 * plugin exits. */
-	struct list_head pending_rpccalls;
-
 	/* If set, the plugin is so important that if it terminates early,
 	 * C-lightning should terminate as well.  */
 	bool important;