RFR(S): 8040807: G1: Enable G1CollectedHeap::stop()

[Per Liden]

Looking for a couple of reviews in this patch.

Summary: This patch re-enables the controlled stopping of G1's 
concurrent threads at VM shutdown. This could potentially cause hangs 
during VM shutdown because the G1 marking threads could get stuck in 
various places and fail to terminate. JDK-8040803 and JDK-8040804 fixed 
these issues, so this is the final step to re-enable the actual stopping 
of those threads. This patch also moves the call to 
CollectedHeap::stop() a few lines down to group the GC related stuff 
together. It also adjusts/removes some comments that are no longer correct.

Bug: https://bugs.openjdk.java.net/browse/JDK-8040807
Webrev: http://cr.openjdk.java.net/~pliden/8040807/webrev.0/

Testing:
- GC nightlies. 5 tests in this suite used to timeout because of the 
issue with hanging threads. They now pass.
- JPRT

Thanks!
/Per