fix(bigquery): reduce default backoffs (#10558)

* fix(bigquery): reduce the initial backoff for job polling

This PR reduces the initial backoff used during job polling from 1s to
50ms.  In practice most time is spent in the hanging poll on the server
side in GetQueryResults, so this reduces opportunities for wasted
waiting.

Fixes: #10555

bigquery/job.go

@@ -356,8 +356,8 @@ func (j *Job) waitForQuery(ctx context.Context, projectID string) (Schema, uint6
 	call = call.FormatOptionsUseInt64Timestamp(true)
 	setClientHeader(call.Header())
 	backoff := gax.Backoff{
-		Initial:    1 * time.Second,
-		Multiplier: 2,
+		Initial:    50 * time.Millisecond,
+		Multiplier: 1.3,
 		Max:        60 * time.Second,
 	}
 	var res *bq.GetQueryResultsResponse