RFR: JDK-8221750: Shenandoah: Enable ThreadLocalHandshake by default

[Roman Kennke]

Back when ThreadLocalHandshake has been introduced, we needed to disable 
it in Shenandoah, because it caused a problem in the serial benchmark, 
together with traversal mode, which would cause the application to hang. 
This has never been explained. However, the problem no longer seems to 
exist, and we shall enable TLHS by default. This also seems to improve 
latency.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8221750
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8221750/webrev.00/

Testing:
The patch has been baked in shenandoah/jdk for a while and undergone our 
rigourous CI testing. I've also run hotspot_gc_shenandoah locally again.

ok?

Roman