v0.2.6

Play a stream TVP's CDN refuses from one of TVP's own servers

TVP hands a stream out at one of its own servers or at a router that
sends on to a CDN, the same one for half a minute at a time. The
router answers the addresses of VPN providers with 403. So behind a
VPN a tune worked or failed by which of the two TVP named just then.

What makes a stream out, its path with the token in it, is good at
either host. A stream that is refused where it was handed out is now
asked for at the host that last served one. A tune with no such host
to go to asks TVP again, for fifteen seconds, until it names one of
its servers.

So that a host is known before anyone tunes, the provider asks for a
stream in the background once it has listed its channels: every ten
seconds, for up to five minutes, until a host serves it. If none
does, the log says that TVP's CDN refuses this address and TVP names
no server of its own. That is how it is behind most Polish servers of
the VPN this was tried with: TVP's own servers do not answer their
addresses either, and nothing in telesfor gets past that. Another
server of the VPN does.

A viewer who leaves while a refused stream is asked for at the host
does not make the provider forget it, and a host that serves no more
does not take with it one that another tune has found meanwhile.

- internal/provider/tvp: the host that serves, learnt from tunes and
  at startup, and its tests.
- docs/usage.md: that not every VPN server in Poland will do.