Issue title: rls cache keys not being used
 Issue body: When caching and RLS are used together with the jinja current_username(True), the username is not added to the cache key

### Expected results

When different users log in to a Superset dashboard having RLS enabled, then the cached visualizations should be specific to _that user_.

### Actual results

While all other cache keys seem to be implemented, the cache keys in the RLS filters seem to be ignored. Therefore, if user A logs in to a dashboard with RLS enabled and refreshes all visualizations, then these visualizations will be cached _without_ the user's RLS filters. Thus, when user B logs in, that user will see user A's cached visualizations.

This could possibly be considered a security issue too, since the wrong data will be shown to the users.

#### Screenshots

n/a

#### How to reproduce the bug

1. Create a new visualization, with caching enabled
2. Add a row level security filter that uses something like 
`username = current_username(True)`
3. Log in as user A, refresh the visualizations
4. Log out as user A and log in as user B. The cached visualizations are from user A

### Environment

(please complete the following information):

I'm using the docker install, last git pull was about a month ago

- superset version: Superset 0.999.0dev (I think it's a heavily updated v0.36)
- python version: 3.6.9
- node.js version: v10.22.0
- npm version: 6.14.6

### Checklist

Make sure these boxes are checked before submitting your issue - thank you!

- [x ] I have checked the superset logs for python stacktraces and included it here as text if there are any.
- [ x] I have reproduced the issue with at least the latest released version of superset.
- [ x] I have checked the issue tracker for the same issue and I haven't found one similar.

### Additional context

I have a solution, will try to issue a PR


The superset logs show that the cache key is being set:

superset_app       | DEBUG:superset.stats_logger:[stats_logger] (incr) set_cache_key

but upon the second user loading the dashboard, it says:
superset_app       | DEBUG:superset.stats_logger:[stats_logger] (incr) loading_from_cache
superset_app       | DEBUG:superset.stats_logger:[stats_logger] (incr) loaded_from_cache

rather than setting its own cache key
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{'source': 'GithubIssues', 'repo_name_with_owner': 'apache/incubator-superset', 'issue_id': 694505882, 'repo_stars': 17967.0}