sql: crdb_internal_mvcc_timestamp for materialized views should update when view is refreshed

[rafiss]

Run the following:

> create table t (a int primary key);
> insert into t values (1);
> create materialized view vw as select a, now() from t;

> select a, now, crdb_internal.approximate_timestamp(crdb_internal_mvcc_timestamp) from vw;
  a |                 now                 | crdb_internal.approximate_timestamp
----+-------------------------------------+--------------------------------------
  1 | 2021-04-01 04:23:04.899462+00:00:00 | 2021-04-01 04:23:04.899462

-- wait

> refresh materialized view vw;

> select a, now, crdb_internal.approximate_timestamp(crdb_internal_mvcc_timestamp) from vw;
  a |                 now                 | crdb_internal.approximate_timestamp
----+-------------------------------------+--------------------------------------
  1 | 2021-04-01 04:26:06.552062+00:00:00 | 2021-04-01 04:23:04.899462

The mvcc timestamp should update, since the row was rewritten.

Epic: CRDB-13625
Jira issue: CRDB-6212

[ajwerner]

This will likely be fixed by the MVCC-compliant AddSSTable.

[rafiss]

@ajwerner is there an issue/PR we can link to?

[ajwerner]

#70429

[rafiss]

#78563 probably fixed this