Mark transaction as open when started async

`txn->open` is not set to `1` on init and there's no handler anywhere to set it on success. In [`Transaction::__construct()`](https://github.com/php/pecl-database-pq/blob/master/src/php_pqtxn.c#L259) it's always set regardless of sync/async.

As handling results of async ops is generally left to the user, I think that simply setting the flag consistently with `Transaction::__construct()` is the right answer here.