Fix layout xml and page layout caching issue on redis cluster under high load

[andrey-legayev]

Description (*)

Bugs which were fixed:

• $this->pageLayout was not checked after reading from cache, but was used as is
• two cache items were used in once place instead of one (performance impact)

Changes:

• replace 2 cache items by 1 - it should improve performance
• add "_MERGED" to cache key suffix to have compatibility with old cache keys during deployment of new version

Fixed Issues (if relevant)

1. Layout corruption with php-fpm (a Magento 1 problem returns) #6942 Layout corruption (Title is wrong, but issue is real)

Manual testing scenarios (*)

N/A

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[m2-assistant]

Hi @andrey-legayev. Thank you for your contribution
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[magento-cicd2]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 3 committers have signed the CLA.

✅ andrey-legayev
❌ magento-team
❌ pdohogne-magento

[andrey-legayev]

I've created a branch from tag 2.3.1 (which is branch "2.2") and pointed pull request to 2.3-develop
I'm not sure why I see not only my commits... can anyone explain it?
Is it a regular practice to create branch from 2.3-develop?

[curto83]

I'm not sure how this works I need help

[andrey-legayev]

I've cherry-picked all my commits to be on top of 2.3-develop and created #22766

[m2-assistant]

Hi @andrey-legayev, thank you for your contribution!
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