Release Note: JEP 351: ZGC: Uncommit Unused Memory

ZGC was enhanced to return unused heap memory to the operating system. This is useful for applications and environments where memory footprint is a concern.

This feature is enabled by default, but can be explicitly disabled using `-XX:-ZUncommit`. Furthermore, memory will not be uncommitted so that the heap size shrinks below the minimum heap size (`-Xms`). This means this feature will be implicitly disabled if the minimum heap size (`-Xms`) is configured to be equal to the maximum heap size (`-Xmx`).

An uncommit delay can be configured using `-XX:ZUncommitDelay=<seconds>` (defaults to 300 seconds). This delay specifies for how long memory should have been unused before it's eligible for uncommit.

For more details, see ([JEP 351](http://openjdk.java.net/jeps/351)).