Improve performance of endpoint for listing existing configuration promotion files

[mariusoe]

In order to list all configuration files which have been modified/deleted/added but not be promoted to the configuration server's live branch, the configuration server provides a HTTP endpoint for fetching them.

This endpoint can become quite slow in case many configuration files are available for promotion. In this ticket, the endpoint's performance should be analyzed and improved, in order to ensure a fast response time of it.