Call SeekDone() to avoid the freeze bug.

This allows constructing an instance in any thread, and register it
inside the IOThread later.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.

Simplifies the code and reduces the number of IOThread roundtrips.

default_value defaults to nullptr.

Fixes deadlock on small responses.

The old struct config_param remains only for top-level string options.

While seeking, metadata must not be updated.  ResponseBoundary() was
added in MPD 0.19.1, but I forgot to add the IsSeeking() check there.
This caused the "seekable" flag to reset.

Fixes playback of redirected streams.

The second time init was called, http_200_aliases pointed to a freed pointer
and leaded to a SEGFAULT.