DBOS.waitFirst (#594)

```python
def wait_first(
        cls,
        handles: List[WorkflowHandle[Any]],
        *,
        polling_interval_sec: float = DEFAULT_POLLING_INTERVAL,
) -> WorkflowHandle[Any]:
```

Take in a list of workflow handles, wait for one to complete, and return
the first to complete.

Also fix an issue with recovery of `DBOS.get_event` and `DBOS.recv`
calls that timeout introduced by
https://github.com/dbos-inc/dbos-transact-py/pull/574.