fix(memcache): Fix comparison of Memcache configs to classes

[nickvergessen]

Summary

In some cases we compared the cache class with a string starting \ and in some others with the ::class which does not have the leading \, so depending on the config value certain bits of functionality would kick in or not.
Now I'm aligning the class names to be without leading \ so we can always compare to ::class

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[nickvergessen]

/backport to stable27

[nickvergessen]

/backport to stable26

[nickvergessen]

/backport to stable25

[ChristophWurst]

🙈

[backportbot-nextcloud]

The backport to stable25 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable25
git pull origin stable25

# Create the new backport branch
git checkout -b fix/foo-stable25

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable25

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport