RFR: 8241361: ZGC: Implement memory related JFR events

[Stefan Karlsson]

Hi all,

Please review this patch to add some memory related JFR events to ZGC.

https://cr.openjdk.java.net/~stefank/8241361/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8241361

Added events:

ZAllocationStall - Record when we run out of heap memory and the Java 
threads stall, waiting for the GC to free up memory.

ZPageAllocation - Updated the existing event to also record the duration 
of the event. Updated the event to only be reported if the allocation 
takes longer than 1 ms.

ZPageCacheFlush - Record when the page cache needs to be flushed. This 
usually happens when we run out of a specific page size and have to 
detach the physical and virtual memory to materialize a new ZPage. We 
also flush pages when we uncommit memory.

ZRelocationSet - Record information about the selected relocation set.

ZUncommit - Record when we uncommit and hand back memory to the OS.

The patch also contains some small cosmetic changes to existing events, 
whitespace fixes.