[DROOLS-512] Re-adding call to setKnowledgeRuntime in StatefulKnowledgeS...

...essionImpl constructor.

It seems as though this call was inadvertently removed
as part of commit 1f8dc32. The method makes a call that
results in the ksession's mbean being created and registered.

Without this call, no ksession mbeans are created.
I just realized that setKnowledgeRuntime(this) results in a second call to create the processRuntime, which is already created in teh constructor. I'll update this PR to ensure the constructor only create the processRuntime once.