We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

This requires the psutil module, and is still opt-in based on the report_stats
config option.

I've added some Documentation, how to get the running Version of a
Synapse homeserver. This should help the HS-Owners to check whether the
Upgrade was successful.