RFR: 8201316: Move G1-related static members from JavaThread to G1BarrierSet

[Per Liden]

As part of the effort to make GCs more pluggable, the G1-specific data 
in JavaThread should be moved out into a more appropriate abstraction.

The first step is to move G1's static members (_satb_mark_queue_set and 
_dirty_card_queue_set) from JavaThread to the G1BarrierSet.

This will be followed up by JDK-8201318, which is a more complex change, 
to remove the non-static G1 members in JavaThread.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201316
Webrev: http://cr.openjdk.java.net/~pliden/8201316/webrev.0

/Per