[Java] Can't add second consumer on same JsonPointer after events were consumed

[ffendt]

Currently, the client doesn't allow a second consumer for the same selection after already events were consumed. Here's how to reproduce the problem:

1. Register consumer A with id "a-id" (+ startConsumption() for the client)
2. Generate events -> A consumes the events
3. Register consumer B with id "b-id"
4. Still only A consumes events
5. Cancel consumer A -> B will now consume events.

Desired functionality:
In step 4, A and B should both consume the same events.

Current implementation:
The client uses the DefaultRegistry class for storing for which JsonPointerSelection which consumers get called. The first time the registry is asked if a JsonPointer matches any of the known selections, it will add the result to a cache. If it is asked again for the same JsonPointer, it will simply return the result of the cache. Since the cache isn't cleared on new registrations, consumer B will never show up in the cached result.

Possible Fix:
This can be fixed by clearing the cache when a new consumer is registered.