Fix unique index + MutTxId::insert not un-deleteting

[Centril]

Description of Changes

Fixes #2134.
The resolution was a fix to a silly bug where we did not un-delete in case of a unique index.
Also fixes an issue in find_same_row_via_unique_index where it didn't check row equality for the found pointer.

API and ABI breaking changes

None.

Expected complexity level and risk

1

Testing

A regression test is added.

Also ran the repro in the issue:

w1$ spacetime start
w2$ spacetime publish basics -s local -c -y
w2$ spacetime call basics add 1 1 -s local
w2$ spacetime call basics add 1 1 -s local
w1$ ^C
w1$ spacetime start
w2$ spacetime sql basics 'select * from boeuf'
 id | n 
----+---
 1  | 1 

[gefjon]

Looks good to me. Thanks for the new test. Once you get CI happy, this is good to merge as far as I'm concerned.