Describe the problem

gc.Info.AffectedKeys is not updated correctly in come cases.

If we have a following data set:

   | a  b  c  d e f g h i j
---+----------------------
 9 |
 8 |
 7 |
 6 |
>5 | A1 B1
 4 |    b2
 3 |    b3
 2 |    b4
 1 |

'>' marks GC threshold.
If batch threshold triggers flushing of GC request at 'b2' (note that iteration is from highest to lowest keys and timestamps) then key 'a' is also counted as affected key.

To Reproduce

Use test data above with gc_test and set threshold to 45 bytes to trigger request at the right mvcc key.

The unfortunate effect of this bug is that tests that assert of Info post GC fail.

Jira issue: CRDB-17502