Raise error if same store used for docs and data

[utf]

Based on the comment below, we should raise an error if the same store is used docs_store and additional_stores

Figured it out - there's some extra configuration stuff that's important here:

1. The regular job store and the addtional_stores.data store need to point to separate JSON files, or the outputs and blob data will be written to the same file. That causes problems because...
2. ...the key for the job store needs to be uuid and job_uuid for blob storage. The default is task_id in maggma

docs_store:
  type: JSONStore
  paths: /Users/aaronkaplan/Downloads/test_output.json
  read_only: False
  key: uuid
additional_stores:
  data:
    type: JSONStore
    paths: /Users/aaronkaplan/Downloads/test_output_blobs.json
    read_only: False
    key: job_uuid

Originally posted by @esoteric-ephemera in #1390