RFR: Here are some Thread cleanup patches

[Martin Buchholz]

The usual story when I change Thread.java is that I'm missing something and
I end up reverting, so I was extra careful this time.

8200122: Remove unused field Thread.threadQ
http://cr.openjdk.java.net/~martin/webrevs/jdk/Thread-threadQ/
https://bugs.openjdk.java.net/browse/JDK-8200122

8200123: Replace Thread.init with telescoping constructor
http://cr.openjdk.java.net/~martin/webrevs/jdk/Thread-init/
https://bugs.openjdk.java.net/browse/JDK-8200123