RFR(xs): 8040245: G1: VM hangs during shutdown

[Per Liden]

Hi,

Could I please have this fix reviewed.

Summary: JDK-8037112 added controlled shutdown of concurrent marking 
threads in G1 during VM exit, to prevent these threads from using 
resources that are destroyed later in the shutdown phase. However, it 
turns out there are at least two unrelated bugs (I've filed JDK-8040803 
and JDK-8040804) in the concurrent mark code which under some 
circumstances can provoke the concurrent mark threads to hang, and thus 
prevent the VM from exiting. This patch temporarily disables the 
controlled shutdown of the CM threads (because it causes timeouts in our 
testing) until those other bugs have been sorted out, at which point we 
should enabled this again (filed JDK-8040807 to track that).

Bug: https://bugs.openjdk.java.net/browse/JDK-8040245
Webrev: http://cr.openjdk.java.net/~pliden/8040245/webrev.0/

Thanks!
/Per