Enhance the retention of jvmstat Monitors.

The original implementation re-extracted all internal jvmstat monitors
from the virtual machine on every exposition request.  This may have
been wasteful, so the monitors are retained between exposition requests
and are periodically updated to include an overall superset.
Merge pull request #21 from prometheus/refactor/jvmstat/retention

Enhance the retention of jvmstat Monitors.