gotwc: Export should be faster

[brendoncarroll]

• The Exporter is currently single threaded, unlike the Importer which is multithreaded.
• Import speeds are faster by a factor approximately equal to the number of cores. Exports could also be this fast.
• Need to check the size from GotFS, then decide whether to export in parallel.

[brendoncarroll]

This is actually a good first issue, since the Importer is in the same package as the Exporter, and provides a blueprint for doing fast imports/exports already.