shared cache: add shared-file support to AXL to parallelize copy

[mcfadden8]

From Adam:
With the current version of AXL, we'd have a single thread on a single compute node copy the entire file from cache to the parallel file system. However, since we're now dealing with a single shared file, the size of the file scales up with the node count of the job. We probably will want to split up a file into regions and assign different portions of the file to be copied by different (multiple) processes. And that seems like it will require us to modify the AXL API so that we can provide an offset and length in addition to the source and destination file names.