RFR: Enable TLHS by default (again)

[Roman Kennke]

A while ago we re-enabled TLHS only disable it again because the Serial
benchmark with Traversal was hanging.

http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-June/006524.html

and:

http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-June/006534.html

I tried today, at least 10 times with both release and fastdebug builds,
with and without traversal, and it did not hang. I also ran the whole
specjvm (with -f3) with adaptive and traversal, plus the
tier3_gc_shenandoah release+fastdebug. No problems.

Do we want to finally enable TLHS for good? Or do we considere too risky
compared to (almost zero immediate) usefulness? It'd allow some more
concurrent processing going forward.

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

Roman