Eliminate dependency on io_thread_get().

Make sure we're using the same EventLoop as the NfsManager.

Eliminate dependency on io_thread_get().

This way, we have four periods instead of the default of two.  With
only two periods, we don't get woken up often enough, and we
frequently encounter buffer overruns.  With four periods, we have more
time to breathe, and the buffer overruns magically disappear.

This setting is mostly useless for capture devices.  There's no point
in configuring it.

This is necessary because we'll never get woken up again by
epoll_wait() after a buffer overrun recovery, unless we start the PCM
explicitly before returning to the I/O loop.

Copy yet more code from the ALSA output plugin.

Copy some good code from the ALSA output plugin.

Fixes assertion failure in implicit destructor.

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.