Fixed #18855 -- Made runserver persist socket across reloads

Using livereload (livereload.com) with Django becomes painful when
updating a file immediately results in reloading the webpage AND
the Django dev server.  There is a short period of time when the dev
server is not listening as it is busy reloading (frequently hit
when using livereload).

This is exacerabated with larger projects as reload time is longer.