cbrestore fails to restore backups with mid transaction data

If a backup with more than 10 elements that has mid transactions documents is restored with cbrestore the restore is likely to fail (it may pass if all the mid transaction documents fall in the last batch of messages to be processed).

It will fail with the message

2020-11-26 12:36:04,934: s0 error: async operation: error: opaque mismatch: 2 3 on sink: http://localhost:8091(b'demo-bucket'@b'127.0.0.1:8091')

Steps to reproduce

1. Stand up a cluster and create a bucket
2. Load 100 documents to the bucket
3. Add some mid-transaction documents see the attachment for a script that does this (the run.sh)
4. Load another 100 documets to the bucket
5. Do a backup with cbbackup
6. Do a restore with cbrestore