Restrict ProjectCacheWorker jobs to one per 15 min

## What does this MR do?

This restricts performing ProjectCacheWorker jobs to once every 15 minutes per project. The goal of this is to reduce disk load in the event of a lot of pushes happening in a short period of time. 

The impact is that cached data (e.g. commit count) may be updated less frequently. Furthermore it could mean that separate push payloads won't lead to e.g. READMEs being updated. Most of the cached data isn't updated very frequently, so this trade off should be worth it considering the alternative is a burning storage layer. 

In the future we'll have to flush caches in a smarter way. For example, refreshing the README cache could probably be done separately from the other caches with its own lease and such. See https://gitlab.com/gitlab-org/gitlab-ce/issues/23550 for more information.

See merge request !7017