Merge branch 'faster-cache-keys' into 'master'

Optimize "cache_key" using a concern

## What does this MR do?

This MR adds a concern (used by Issue and Note) that provides an optimized version of Rails' `cache_key` method. See 77c8520e2ecd70520757aed0fbdf434643b60234 for more details.

## Are there points in the code the reviewer needs to double check?

No, though a spell check would be appreciated.

## Why was this MR needed?

When loading a lot of data from Redis (e.g. an issue with lots of notes) quite a large amount of time is spent in generating cache keys. This is due to multiple reasons such as:

* Rails trying to figure out if it should use `updated_at` or `updated_on` using somewhat inefficient code
* Rails relying on pluralization logic to figure out how to generate a cache namespace using a model name
* Rails calling a whole bunch of methods in general in the process of generating cache keys

In short, Rails is trying to cater to every possible use case, at the cost of performance.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13651 is not directly related but I ran into this `cache_key` problem when looking into said issue.

See merge request !5715

1 files changed, 1 insertions, 0 deletions

diff --git a/CHANGELOG b/CHANGELOG
index ec2df962675..184704d8594 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -26,6 +26,7 @@ v 8.11.0 (unreleased)
   - Clean up unused routes (Josef Strzibny)
   - Fix issue on empty project to allow developers to only push to protected branches if given permission
   - Add green outline to New Branch button. !5447 (winniehell)
+  - Optimize generating of cache keys for issues and notes
   - Improve performance of syntax highlighting Markdown code blocks
   - Update to gitlab_git 10.4.1 and take advantage of preserved Ref objects
   - Remove delay when hitting "Reply..." button on page with a lot of discussions