Improve Globus transfer concurrency

[lukaszlacinski]

With --non-blocking option, zstash submits a Globus transfer and immediately creates a subsequent tarball. Zstash does not wait until the transfer completes to start creating a subsequent tarball. On machines where zstash tarballs are created faster than they are transferred to a remote endpoint, zstash will create multiple Globus transfers that are put in a queue by the Globus service. Zstash should submit transfers to transfer as many zstash tarballs as have been created at the moment. On machines where it takes more time to create a tarball than transfer it, each Globus transfer will have one file. On machines where it takes less time to create a tarball than transfer it, the first transfer will have one file, but the number of tarballs in subsequent transfers will grow finding dynamically the most optimal number of tarballs per transfer.

[forsyth2]

Resolved by #214.