* Cache user assignments to avoid unnecessary DB queries
alternative to #9138 and #10981, only cache on `checkAccess` call which is usually
called on every request. Cache is not necessary in RBAC management.
Similar to #14061 but includes proper cache invalidation and test.
`getAssignments()` always queries the DB. The cache is only applied on
`checkAccess` calls, and invalidated as soon as the RBAC structure is
modified through the manager component (verified by the test case).
Regarding [concerns of memory usage](https://github.com/yiisoft/yii2/pull/14061#issuecomment-297982502)
if used in batch mode on multiple users, you can call
`invalidateCache()` method if this really causes a problem.
fixes#7743close#9138close#14061close#10981
See also
- https://github.com/yiisoft/yii2/issues/7626#issuecomment-77745166
- https://github.com/yiisoft/yii2/pull/14061#issuecomment-319645488
* improve test naming
* fix tests
* fix assignment cache for non-scalar user-ids
