WIP - DONOTSUBMIT

This commit introduces HotSpot JVM monitoring through the low-level
"jvmstat" interfaces, which are exposed through the MMAPed "HSPerfData"
file.  jvmstat offers much more detailed analysis of the internals of
VM health than MX management beans or even JMX ever could.

= TODO Before Final Review
- Implement metric visitor.
- Expose survivor space agetable cohort metrics as canonical example.