RFR: WB slowpath should assist with evacuation of adjacent objects

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/wb-assists/webrev.01/

This small patch introduces the experimental way to amortize the costs of entering the WB slowpath,
but evacuating adjacent objects as well. This was shown to be effective in WB-intensive workloads.
The default value is chosen as the tradeoff between the additional latency introduced by the
barrier, and the throughput wins. With current setting, we spend ~500 ns for assists.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey