[DocDB] Reduce cost of point lookup reads in colocated tables from table level tombstones

[bmatican]

Jira Link: DB-5839

Description

As a followup to #15206, we fixed the correctness part of the table-level tombstone handling, but are now even more explicitly paying the cost of an extra seek, to the table-level tombstone key, for every single point lookup read operation.

We should figure out either some caching or persistence change, to not have to pay the cost of this extra docdb Seek operation, every time.

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