Removed session caching from the UserDiscriminator

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes, constructor of UserDiscriminator has changed
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none, related to #12 

This removes the session-cache from the UserDiscriminator,
first the UserDiscriminator should not cache this and second because it gives some issues when the session-date is shared between users.

Caching of the discriminated user-system will be reintroduced in another PR, but must be enabled explicitly then.
Also will that introduce a new event 'USER_DISCRIMINATED' to inform that the active user-system has changed.