Modifications needed for the migration to cozy

[shepilov]

Results of the migration:

• List of the migration by user:
  For each migration, we need to have:
  • Number of the migrated files
  • Total number of files for the user
  • Number of the migrated files
  • Number of errors
    For every error, we need to have the type of error(if we can determine)
    • stacktrace
    • input data that caused an error (user, doc id, file id, etc)

List of the errors that we know and need to deal with:

• DNS resolution of the user's hostname
• Unknown errors with the empty response from the cozy stack
• Files don't exist in S3
• Metadata is corrupted(undefined 'id' field)

To run several migrations in parallel, we need to split the user list into batches.
To be simple and not use a distributed lock on the user, we can continue to consume users from command params, but we need to pass a lot of emails as well as it's not so comfortable to do with params, so let's also introduce env variable for the users list for the migrations.