RFR: Use PLAB for evacuations instead of TLAB

[Roman Kennke]

I propose to use PLAB for evacuations (write-barrier and GC workers). It
has the following benefits:
- It reverts/removes a *whole* lot of code to upstream state, thus
significantly improving/reducing our upstream exposure
- It is a much better fit for what we need (e.g. already has rollback
functionality)
- It avoids disturbing TLAB stats by GCLAB activity (which has much
different characteristics)
- I implemented it to not mix up mutator and collector GCLAB stats,
which have very different characteristics (churn vs. very occasional allocs)

I hope I got the mechanics right. This might need some experimentation
and tweaking afterwards, and would then be backportable.

http://cr.openjdk.java.net/~rkennke/plab-gclab/webrev.00/

Testing: hotspot_gc_shenandoah ok