JCR-2768 Replace finalize in SessionImpl with Phantom references

This patch removes the finalize function in SessionImpl. The garbage collection runs much smoother now.
Tested with 20 threads that open 20000 sessions each and create 50 nodes. Images are from https://github.com/chewiebug/GCViewer.


Original code:
![orig](https://cloud.githubusercontent.com/assets/7127703/2641069/9c8f7b48-bef3-11e3-9efd-7d7ddd5da3e8.png)

Patch:
![patched](https://cloud.githubusercontent.com/assets/7127703/2641070/9c916df4-bef3-11e3-9c62-fb2ac98cc399.png)