Provide Exposition of "jvmstat" Metrics

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.  This is to say, most of the data one really should care about for VM health and code regression finding is exposed only through this interface.