utils: add a global tmpctx.

I did a brief audit of tmpctx uses, and we do leak them in various
corner cases.  Fortunely, all our daemons are based on some kind of
I/O loop, so it's fairly easy to clean a global tmpctx at that point.

This makes things a bit neater, and slightly more efficient, but also
clearer: I avoided creating a tmpctx in a few places because I didn't
want to add another allocation.  With that penalty removed, I can use
it more freely and hopefully write clearer code.

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

common/io_debug.c

@@ -11,9 +11,7 @@ int debug_poll(struct pollfd *fds, nfds_t nfds, int timeout)
 	if (t)
 		errx(1, "Outstanding taken pointers: %s", t);
 
-	t = tmpctx_any();
-	if (t)
-		errx(1, "Outstanding tmpctx: %s", t);
+	clean_tmpctx();
 
 	return poll(fds, nfds, timeout);
 }