Summary
Currently wiredtiger alter is not transactional. In MongoDB, when we convert indexes to unique or non-unique, the change to the index table metadata by alter will become visible right away before the changes to the catalog commits, causing a short window of inconsistency.

Motivation
This affects index uniqueness conversion in the server code. A workaround SERVER-68186 is investigated and it shouldn't be blocked by this.

• How likely is it that this use case or problem will occur?
  This is an edge case. Only a checkpoint + shutdown or fcbis opening a backup cursor during the inconsistent window will cause this issue.

• If the problem does occur, what are the consequences and how severe are they?
  The server will fassert and crash at startup but after SERVER-68186 it should be mitigated.

• Is this issue urgent?
  This is not urgent.

Acceptance Criteria (Definition of Done)
The update to the index table metadata and the update to other tables' contents are atomic.

• Testing
  (What all testing needs to be done as part of this ticket? Unit? Functional? Performance?Testing at MongoDB side?)

• Documentation update
  (Does this ticket require a change in the architecture guide? If yes, please create a corresponding doc ticket.)

[Optional] Suggested Solution
(Is there any suggested solution to handle this issue? Is it related to any existing WT ticket? Is it related to any previous issue fixed? If yes, link the WT ticket number using related to, depends on, dependent on by links)