RFR(xs): 8134744: WorkerThread::id() should match GangWorker's worker_id

[Per Liden]

Hi,

This is a follow up patch to JDK-8134749 (SoftReferences declared dead 
too early), which makes sure WorkerThread::id() and match GangWorker's 
worker_id always match. This allows them to be used interchangeably, 
which is both convenient and removes a potential source for bugs (it's 
easy to assumed they are the same). This change should not affect any 
existing code.

Webrev: http://cr.openjdk.java.net/~pliden/8134744/webrev.0/

Bug: https://bugs.openjdk.java.net/browse/JDK-8134744

Testing: jprt

/Per