When we open a backup cursor and return documents via the $backupCursor aggregation command, documents currently have the following formats depending on the type of backup.

• Non-incremental (i.e. full) backups return one document per file with the format:

  {
      filename: String,
      fileSize: Number
  }

• The first full backup used as the basis for future incremental backups returns one document per file with the format:

  {
      filename: String,
      fileSize: Number,
      offset: Number (=0),
      length: Number (=fileSize)
  }

  Note: In this case, offset=0 and length=fileSize always holds.

• Incremental backups return one document per unchanged file with the format:

  {
      filename: String,
      fileSize: Number
  }

• Incremental backups return multiple documents (one document per block, where each block has a default size of 16MB) per changed file with the format:

  {
      filename: String,
      fileSize: Number,
      offset: Number,
      length: Number
  } 

We should make this format more consistent by returning document with the following format regardless of the type of backup.

{
    filename: String,
    fileSize: Number,
    offset: Number,
    length: Number
}

In the case of non-incremental backups and the first full backup used as the basis for future incremental backups, offset=0 and length=0 would clearly indicate the type of backup. Since this involves additive changes to the backup API, it should require few changes from downstream teams like Cloud Backup.