Track native memory usage via jcmd

https://lists.opendaylight.org/pipermail/controller-dev/2017-November/014037.html

add the following magic incantations to the JVM options,
e.g. by editing the bin/karaf script:

DEFAULT_JAVA_OPTS="$DEFAULT_JAVA_OPTS -XX:+HeapDumpOnOutOfMemoryError
-XX:NativeMemoryTracking=detail -XX:+UnlockDiagnosticVMOptions
-XX:+PrintNMTStatistics -XX:-AutoShutdownNMT"

BUT the Kernel OOM killer will do SIGKILL (kill -9) so AutoShutdownNMT most likely won't work, but if we could obtain native memory usage during normal runtime via the following command, if you manage to run that some time (shortly) before it dies, that would still be interesting to see:

jcmd <PID> VM.native_memory detail scale=MB