RFR: 8240745: Implementation: JEP 377: ZGC: A Scalable Low-Latency Garbage Collector (Production)

[Per Liden]

Please review the patch for JEP 377: ZGC: A Scalable Low-Latency Garbage 
Collector (Production).

This patch changes the UseZGC option, some of the ZGC-specific options, 
as well as some of the ZGC-specific JFR events from experimental to 
product. It also adjusts tests using ZGC to not supply 
-XX:+UnlockExperimentalVMOptions.

Note that this patch builds on JDK-8241361, which as of this writing, 
has not yet been pushed.

JEP: https://openjdk.java.net/jeps/377
Bug: https://bugs.openjdk.java.net/browse/JDK-8240745
Webrev: http://cr.openjdk.java.net/~pliden/8240745/webrev.0

Testing: Passed tier 1-7 on all platforms.

/Per