Allow falling back to a non-SASL connection on DataTransferProtocol in several edge cases.

HDFS-2856 implemented general SASL support on DataTransferProtocol. Part of that work also included a fallback mode in case the remote cluster is running under a different configuration without SASL. I've discovered a few edge case configurations that this did not support:

• Cluster is unsecured, but has block access tokens enabled. This is not something I've seen done in practice, but I've heard historically it has been allowed. The HDFS-2856 code relied on seeing an empty block access token to trigger fallback, and this doesn't work if the unsecured cluster actually is using block access tokens.
• The DataNode has an unpublicized testing configuration property that could be used to skip the privileged port check. However, the HDFS-2856 code is still enforcing requirement of SASL when the ports are not privileged, so this would force existing configurations to make changes to activate SASL.

This patch will restore the old behavior so that these edge case configurations will continue to work the same way.