Refactor zstash

[forsyth2]

Extremely early, experimental draft of what a zstash refactor would look like. Specifically, the refactor would store as much state as possible in an object rather than passing around many variables (especially global variables).

[forsyth2]

Advantages:

• State could be easily "snapshotted". In particular, it would be easier to track state of Globus transfers (as in Non block testing fix #363).
• Easier handling of HPSS (e.g., the HPSSType Enum of this PR allows us to immediately see which HPSS scenario we're in rather than repeatedly parsing the HPSS path

Obstacles:

• Unclear at the moment what Config class is for and if the CommandInfo of this PR would be better off combined with it
• Extensive changes to parameter passing.