Add retries to upload logic

When uploading large amounts of data to S3, we occasionally see failures where the AWS sdk tries to use a closed network connection. The upstream bug appears to be aws/aws-sdk-go#3406.  I'm not sure why the error manifests but it's causing us significant pain. Rather than retry the entire base backup, we'll retry the WAL segment upload.

```
ERROR: 2020/08/06 08:39:52.198782 failed to upload 'basebackups_005/base_0000000100006F04000000E4/tar_partitions/part_19148.tar.br' to bucket 'S3_BUCKET':
    MultipartUpload: upload multipart failed
caused by: RequestError: send request failed
caused by: Put https://S3_BUCKET/basebackups_005/base_0000000100006F04000000E4/tar_partitions/part_19148.tar.br?partNumber=2:
    write tcp 10.64.18.161:42118->52.216.134.19:443: use of closed network connection
ERROR: 2020/08/06 08:39:52.198805 upload: could not upload 'base_0000000100006F04000000E4/tar_partitions/part_19148.tar.br'
ERROR: 2020/08/06 08:39:52.198818 failed to upload 'basebackups_005/base_0000000100006F04000000E4/tar_partitions/part_19148.tar.br' to bucket 'S3_BUCKET':
    MultipartUpload: upload multipart failed
caused by: RequestError: send request failed
caused by: Put https://S3_BUCKET/basebackups_005/base_0000000100006F04000000E4/tar_partitions/part_19148.tar.br?partNumber=2
    write tcp 10.64.18.161:42118->52.216.134.19:443: use of closed network connection
ERROR: 2020/08/06 08:39:52.198833 Unable to complete uploads
```

internal/uploader.go

@@ -100,12 +100,18 @@ func (uploader *Uploader) Upload(path string, content io.Reader) error {
 	if uploader.tarSize != nil {
 		content = &WithSizeReader{content, uploader.tarSize}
 	}
-	err := uploader.UploadingFolder.PutObject(path, content)
-	if err == nil {
-		return nil
+	// Add retries to work around https://github.com/aws/aws-sdk-go/issues/3406
+	const retries = 3
+	var err error
+	for i := 0; i < retries; i++ {
+		err = uploader.UploadingFolder.PutObject(path, content)
+		if err == nil {
+			return nil
+		}
+		tracelog.ErrorLogger.Printf(tracelog.GetErrorFormatter()+"Retrying upload error:\n", err)
 	}
+	tracelog.ErrorLogger.Printf(tracelog.GetErrorFormatter()+"Exhausted upload retries:\n", err)
 	uploader.Failed.Store(true)
-	tracelog.ErrorLogger.Printf(tracelog.GetErrorFormatter()+"\n", err)
 	return err
 }