Create plug-able backend for the replay files

Currently all replay files are stored on local disk. This work for most use cases as these file don't represent a large amount of data. It would be better to allow these files to be stored elsewhere, possibly a distributed filesystem or the like.

Pchyderm's PFS could be well suited for this use case.

Start by locating the code that stores and reads replay files, then trace how local-disk paths are passed through the replay workflow. Define the storage-backend boundary and verify that local storage remains supported while an alternate or distributed backend can be supplied; the issue names Pchyderm's PFS as a possible option.

Written by the indexing model from the issue text.