Add the option to use Symfony locks

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**Is your feature request related to a problem? Please describe.**
When a user uploads a pretty large image that takes some time to generate a thumbnail for and said user keeps on hitting f5 on the page because the image is not loading (it is being generated) multiple processes try to generate the thumbnail, one for each reload. Because the image processing on its own can eat up RAM quite well you can run into the issue that you php-fpm process gets killed to recover memory, because otherwise the server would crash

**Describe the solution you'd like**
Add the option to use the [Symfony lock component](https://symfony.com/doc/current/components/lock.html) so each image can only be processed by exactly one process at a time

**Describe alternatives you've considered**
I though about triggering the generation on uploading the image and that might help, but it doesn't really solve the issue

**Additional context**
https://github.com/MbinOrg/mbin

Start by reviewing the bundle's thumbnail-generation flow and the Symfony Lock component documentation linked in the issue. Determine where concurrent processing for the same image is initiated. Done means the bundle offers an option that ensures only one process generates a given image at a time, while other requests avoid duplicate processing.

Written by the indexing model from the issue text.