Add protection on Archive that avoids the Duplicate batch issue

[tiagoicp]

No description provided.

[tiagoicp]

Fixes Issue: #23

[tiagoicp]

Opened an issue on Mops to better understand the desired new behaviour.
caffeinelabs/mops#195

[tiagoicp]

Hi @tomijaga , think this is ready for review.

I have added specs that showcase it's allowing to be duplicated. The original problem is exposed on the issue (#23), and in short, because it's an intercanister call, you may have multiple transactions in Ledger while it waits for the callback that clears the SB (Stable Buffer - transactions).

So, this is an issue that typically arises in a peak transactions moment.

This fix was tested, it's not greedy, and tried for it to be fully compatible to any state of the archive. Example: that it already has duplicated txs, and therefore can't use total_txs - 1 to be the last_tx_index.

Demo of tests failing and passing: