hg: jdk/sandbox: Cache native thread id to avoid call to JNI in blocking I/O ops

[alan.bateman at oracle.com]

Changeset: 1a13164f3f4f
Author:    alanb
Date:      2019-02-05 18:53 +0000
URL:       http://hg.openjdk.java.net/jdk/sandbox/rev/1a13164f3f4f

Cache native thread id to avoid call to JNI in blocking I/O ops

! src/java.base/share/classes/java/lang/System.java
! src/java.base/share/classes/java/lang/Thread.java
! src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
! src/java.base/unix/classes/sun/nio/ch/NativeThread.java
! src/java.base/unix/native/libnio/ch/NativeThread.c