ZGC parsing refactoring

[kcpeppe]

1. Apply a DRY refactoring to the ZGC class hierarchy.
2. Capture some information not currently captured such as work thread counts.

[jlittle-ptc]

The current ZGC implementation also opts to use timestamp differences rather than parsed information for event durations. For legacy logs (No -XX:+ZGenerational), durations don't seem to be printed out, so timestamp differences will likely need to be used as a fallback.