Real-time planning: memory leak when calling addProblemFactChange(ProblemFactChange)

When using Planner with DroolsScoreDirector, after each problem fact change, number of planning entity instances increases.

Steps to reproduce:
1. open cloud balance example with GUI, choose the largest input
2. start solving the problem
3. delete some of the listed computers - to make the fact change (for me, 5 computers in a 5 seconds are enough)
4. take a heapdump (I used jmap -dump:live,file=... to enforce GC first)
5. open visualvm or similar tool and look for the planning entity class
6. repeat 3. - 5. several times, you should see increasing number of planning entity instances

Also, there is a rule base holding sessions under the Planner API - and number of sessions also increases in time (these sessions reference to planning entity instances).

Steps to reproduce:
1. open the last heapdump in similar tool and look for ReteooRuleBase.
2. the rule base holds ObjectHashSet with stateful sessions in its entries
3. you should see a different number of these session in every heapdump taken in previous recipe