Implement server-side rendering

Add an additional view helper `react_server_component` with the same interface/functionality as `react_component` with the exception that the body of the tag will be filled with a server rendering of the HTML markup. (Im not sure if it should be a different method name or perhaps just an argument to `react_component`? )

For thread-safety we use the ConnectionPool class from @mperham to control access to the JS VMs.  It is performing well under load in my tests.