RFR: Abstraction for TLAB dummy object

[Roman Kennke]

This changes/implements a proper abstraction for TLAB filler objects. It
mirrors what I am going to propose for:

https://bugs.openjdk.java.net/browse/JDK-8204685

.. but with Shenandoah implementation.

It also fixes an issue: currently when filling PLABs, we zap the filler
object with zeroes. We don't need to do that, and save those memory
accesses.

http://cr.openjdk.java.net/~rkennke/tlabfiller/webrev.00/

Testing: tier3_gc_shenandoah ok

Ok?

Roman