restore: ensure the ALTER TABLE step is also serial

[kennytm]

Prevent issue like TOOL-633

What problem does this PR solve?

Ensure the ALTER TABLE step is serial to prevent two DDL be executed concurrently which may cause TOOL-633.

What is changed and how it works?

Add a mutex around the ALTER TABLE step

Check List

Tests

Code changes

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[kennytm]

/run-all-tests

[kennytm]

@GregoryIan PTAL

cc @winkyao

[IANTHEREAL]

LGTM

[iamxy]

/ok-to-test

[iamxy]

LGTM