ENG-5632:  Add the following to @SystemInformation:
- STARTTIME, which is the milliseconds-since-the-epoch at which the
  cluster started
- UPTIME, which is a human-readable string containing the cluster (not
  node) uptime.
ENG-5632: Make @SystemInformation OVERVIEW contain all the keys in the
separate JMX SystemInformation thing.
Merge pull request #1239 from VoltDB/eng5632

Eng5632