(PDB-711) Add factsets endpoint to V4 PuppetDB API

This pull request adds the /factsets endpoint specified in the
"Structured Facts Design and Plan" document circulated to Puppet-dev
on June 30.  The response is a list of maps, each describing the
facts, environment, and time of most recent fact submission for
an agent node.
check which ones is faster, the one in utils or this one, and throw away the rest.