On user login, we were aggressively clearing the channel member
cache when it wasn't necessary. There is no channel membership
info that is changing on a user login.
Additionally, we fix some more issues with pointer passing
to the cache. It's a known problem that our value passing
style isn't consistent when we fetch items from cache.
Sometimes we pass pointer-to-pointer, sometimes it's just a
pointer. This is also the reason for https://github.com/mattermost/mattermost/pull/27830.
We fix 2 such cases where we passed pointer-to-pointer
but didn't handle the special case inside the cache.
This time, we actually fix it in the app layer instead
of the cache layer and also add a test for good measure.
https://mattermost.atlassian.net/browse/MM-60480
```release-note
NONE
```
Co-authored-by: Mattermost Build <build@mattermost.com>
d45a54a8e9