limit concurrency to 1 for no-parallelism transfers (SOFTWARE-1495)

Forcing gridftp-hdfs to limit the in-flight transfers to 1 prevents the
out-of-sequence corruptions seen in SOFTWARE-1495.

This also works around the issue mentioned in the ticket, where
globus-url-copy would always hang after the tranfer completed.

This corresponds to 1495-optimal-concurrency.patch used in osg.