[PLAT-108920] not skip samples during dedup

[jnyi]

A lot of unit tests broken to be fixed, but the idea is currently both iters proceed and potentially skip data points in between. Instead, we only scan iterators and move forward if previous value is being used.

For example:
A: 1, 4, 5
B: 1, 2, 3, 4, 5

Before the change: dedup'ed result will be 1, 4, 5

After the change, the result will be 1, 2, 3, 4, 5

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Verification

[jnyi]

closing this one, we'd better not to change existing behavior and introduce another merge dedup iterator, fyi @hczhu-db