server: Initial attempt to use http.ServeContent.

Using http.ServeContent allows us to not reimplement http serving logic
(range, cache, response codes, etc.).
Currently using a hack where I know Seek is only used to determine file
size if Content-Type is already set; need to generalize and fix that.
It fails for large files that start using Content-Range stuff.
Disable debug sleep for 2 seconds.
Use time-based seed for rand.
Don't use http.ServeContent until io.ReadSeeker is correctly implemented.