RFR (M): 8168467: Use TaskEntry as task mark queue elements

[Thomas Schatzl]

Hi all,

  can I have reviews for this cleanup that changes the oop-misuse for
marking (setting LSB to 1 for array slices) into adding a
proper G1TaskQueueEntry that hides that mess a little?

CR:
https://bugs.openjdk.java.net/browse/JDK-8168467
Webrev:
http://cr.openjdk.java.net/~tschatzl/8168467/webrev/
Testing:
jprt, tier2+3 gc tests

Thanks,
  Thomas