Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Don't access AudioOutput::pause.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes a potential deadlock introduced by commit 94528735

Eliminate dependency on io_thread_get().

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

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