and objects were byte arrays (typically 8KB).
limit it to 64, so typically not more than 512kb is used. doesn't change
the max speed when downloading on localhost, and decreases the RSS usage
with slow clients.
* renamed `run_tar_in_thread()` to `stream_tar_in_thread()`
* use `impl Responder` instead of `Result<HttpResponse>`
* add an `.unwrap()` to `StaticFiles::new()` (seems to be an upstream API change)
… instead of unbounded() with no limits in buffering
this should implement a back pressure, when the client can't read the
response fast enough, the thread creating the archive would block the
write, instead of filling the memory.
Use Sink::wait() to make a blocking Sender
