changefeedccl: db-level feeds: use fixed timestamp for descriptor fetches #154549
Description
When a changefeed is planned and executed, there are several places where target table descriptors are fetched. With a db-level changefeed, the set of tables can change during changefeed startup. The table descriptors should be fetched at a fixed time, when the changefeed is starting from.
Jira issue: CRDB-54992
Epic CRDB-1421