Loading `ApplicationSetting` from Redis was responsible for at least 50%
of the CPU load of the Redis cluster on GitLab.com. Since these values
generally don't change very much, we can load this from the database and
cache it in memory, skipping Redis altogther. We use
`ActiveSupport::Cache::MemoryStore` as a drop-in replacement for
`RedisCacheStore` even though we probably don't need synchronized access
within `Thread.current`.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63977
Caching the attributes as JSON and manually instantiating the record
ended-up very complex since the edge-cases such as upload fields,
serialized fields, and fields with custom accessors had to be handled.
To ensure 3 points out of 4 are checked from https://gitlab.com/gitlab-org/gitlab-ce/issues/45175
we now include the Rails version in the cache key.
Signed-off-by: Rémy Coutable <remy@rymai.me>
