Remove the use of Ctid in merge

[nopcoder]

Current merge implementation uses diff result which communicates the source entry information using CTID.
In order to reduce the risk that ctid will not be valid at the point of the merge, the diff implementation should communicate the captured entry at the point of the diff.

[nopcoder]

Based on "lakeFS on the Rocks - Design Document" we are planning to stop using Postgres as a metadata source for committed information.