RFR: 8205024: ZGC: Worker threads boost mode not always enabled when is should be

[Per Liden]

There's a race when deciding if worker threads should be boosted (very 
similar to the race in JDK-8205022), which can result in worker threads 
not being boosted when they should. The effect of this is that the GC 
cycle takes longer to complete, which in turn means that stalled threads 
wait longer than they would have done otherwise.

The solution to this is very similar to the solution to JDK-8205022.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205024
Webrev: http://cr.openjdk.java.net/~pliden/8205024/webrev.0

Testing: tier{1,2,3,4}

/Per