Upload unification & upgrades

[gsfr]

Edited by @kofalt - itemized list:

1. Lay initial groundwork for upload unification. (@kofalt)
2. EnginePlacer is written but needs to be rigged up and connected to the endpoint. (@nagem)
3. LabelPlacer and (new) UidPlacer should replace most logic currently in upload.reaper and upload.uploader. (@rentzso)
4. After that is complete, consider unifying the LabelPlacer & UidPlacer. (@gsfr)
5. Generate closures for on-the-fly processing; performance optimization. (@kofalt)
6. Once all the above is complete, consider input schema unification(@gsfr)
7. Once all the above is complete, consider Placer naming changes (@kofalt)

[kofalt]

It's my understanding that the UIDPlacer and LabelPlacer are complete, ticking that box. The code basically is unified right now via subclassing, so ticking that box. We punted on closure generation, and naming turned out to be fine, so ticking those boxes. Editing final placer box to be unassigned. Moving ticket back to @gsfr for prioritization of the last items :)