bigquery: ArrowIterator Next is not thread-safe and might be leaking goroutines

[alvarowolfx]

Per discussion with customer, we found points of issues:

• ArrowIterator.Next is not Thread-safe
  • init call needs locks
  • first <-it.records call can be potentially removed
• if multiple errors lock the err channel, context is not checked
• consumeStream needs to check context done
• double check that only one context is being used (it.ctx / it.session.ctx )