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>

openingd/opening.c

@@ -195,7 +195,8 @@ static u8 *opening_read_peer_msg(struct state *state)
 				    &state->channel_id,
 				    sync_crypto_write_arg,
 				    status_fail_io,
-				    state)) == NULL);
+				    state)) == NULL)
+		clean_tmpctx();
 
 	return msg;
 }
@@ -772,6 +773,7 @@ int main(int argc, char *argv[])
 	status_trace("Sent %s with fd",
 		     opening_wire_type_name(fromwire_peektype(msg)));
 	tal_free(state);
+	tal_free(tmpctx);
 	return 0;
 }
 #endif /* TESTING */