title: A background URLSession with identifier already exists
body: <p>I have an S3Service which is a singleton that manages all the S3 related uploads and downloads.
  When I upload the first image it works fine but if I try to upload an Image consecutively It gives me this warning and the completion block never gets called.</p>
  
  <blockquote>
    <p>A background URLSession with identifier <code>com.amazonaws.AWSS3TransferUtility.Identifier.TransferManager</code> already exists.</p>
  </blockquote>
  
  <p>This is how I upload method looks:</p>
  
  <pre><code>if let data = image.jpegData(compressionQuality: 0.5) {
          let transferUtility = AWSS3TransferUtility.s3TransferUtility(forKey: S3Service.TRANSFER_MANAGER_KEY)
          transferUtility.uploadUsingMultiPart(data: data, bucket: EnvironmentUtils.getBucketName(), key: filename, contentType: "image/jpg", expression: nil, completionHandler: { task,error in
  
              if let error = error {
                  print(error.localizedDescription)
              } else {
                  print("Image upload success")
              }
          })
  }
  </code></pre>