mod_gridfs is dead, long live gridfs_server

Long time no write, but mod_gridfs is no longer going to be developed, it's being superseded by gridfs_server. It's written in C#, based on ASP.NET Core, Kestrel, runs on .NET Core 2.0 Preview 2, and successfully serves about 25 million files per day from our MongoDB GridFS installation, as a backend to a front-end cache (that serves about 450 million files per day).