Missing retry during ExecuteStreamingSql call in _restart_on_unavailable

[harshachinta]

The _restart_on_unavailable method restarts an iteration in case of an UNAVAILABLE error. However, the initial RPC call for ExecuteStreamingSql/ExecuteStreamingRead to get the iterator may also fail with an UNAVAILABLE error, and this retry is not handled. Consequently, errors are surfaced to the customer where the intended behavior is to retry the requests.
One example of this issue is #1150