Check for lock existence before unlinking

My logs are filled with a bazillion errors stating "Warning: unlink(/var/www/mysite/app/cache/prod/http_cache/md/cf/47/c693da5dab3eccb65fa36a9b4b07ad0f7cc4.lck): No such file or directory in /var/www/mysite/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpCache/Store.php line 53"
bug #9207 [HttpKernel] Check for lock existence before unlinking (ollietb)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9207).

Discussion
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[HttpKernel] Check for lock existence before unlinking

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

My logs are filled with a bazillion errors stating "Warning: unlink(/var/www/mysite/app/cache/prod/http_cache/md/cf/47/c693da5dab3eccb65fa36a9b4b07ad0f7cc4.lck): No such file or directory in /var/www/mysite/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpCache/Store.php line 53"

Commits
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89cd3b9 Check for lock existence before unlinking
fixed CS