bandwith throttling detection

We've got reports on OpenVPN bandwidth throttling and we had no of-the-shelf method to measure it.

Current openvpn nettest does not account bandwidth and provides no good way to run a control bandwidth measurement.

One of the possible ways is to implement test helper: OpenVPN server that provides no connectivity besides a connection to a localhost-hosted website serving several huge static files of random data (zero-filled files may be compressed in transit and skew the results).

We can attempt to download same file via various network routes:

• http / https
• Internet / OpenVPN + udp / OpenVPN + tcp / tor / hidden-service.onion / psiphon / etc.
• standard / non-standard OpenVPN ports

This test should also produce sane results despite the fact there may be several concurrent users running the test, so test helper should also account it's own bandwidth, monitor and/or prevent overloading.