Disable response buffering for /admin/logs (#197)

This patch disables buffering for the streaming endpoint /admin/logs by
setting the special nginx proxy response header X-Accel-Buffering to
"no". Even though the default buffering settings seems to work fairly
well without too many buffer hiccups it is recommended to disable
buffering for this type of streaming endpoints.

As an alternative, buffering can be disabled in the nginx config by
introducing a new /admin/logs location block, but since the scope here
is very narrow it seems much simpler to just set the header.

src/admin.jl

@@ -104,6 +104,9 @@ function handle_admin_authenticated(http::HTTP.Stream)
         color = get(params, "color", "true") == "true"
         req_level = get(params, "level", "info") == "debug" ? Logging.Debug : Logging.Info
         HTTP.setheader(http, "Content-Type" => "text/plain")
+        # Tell nginx to not buffer the response, see
+        # https://www.nginx.com/resources/wiki/start/topics/examples/x-accel/#x-accel-buffering
+        HTTP.setheader(http, "X-Accel-Buffering" => "no")
         HTTP.startwrite(http)
         attach(ADMIN_LOGGER) do level, message
             level < req_level && return