Add server_multithreaded parameter

Puppet Server 6.8 has just been released with an experimental new
feature.

This release adds a new JRuby pool architecture that maintains a
single JRuby instance through which requests to Puppet Server are run
concurrently. In this mode, the server's memory footprint is
significantly lighter, because it no longer needs to run multiple JRuby
instances. Toggle this behavior by setting the
jruby-puppet.multithreaded to true.

This commit adds support for this new setting.