RFR (S): 8238160: Uniformize Parallel GC task queue variable names

[Thomas Schatzl]

Hi all,

   can I have reviews for this small change that moves some global 
typedefs used only by Parallel GC from taskqueue.hpp to parallel gc 
files, and further makes naming of instances of these more uniform?

CR:
https://bugs.openjdk.java.net/browse/JDK-8238160
Webrev:
http://cr.openjdk.java.net/~tschatzl/8238160/webrev/
Testing:
local compilation

Thanks,
   Thomas