Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

The "seeking" flag is not set for the initial seek, and so
decoder_read() could be canceled when another SEEK was emitted during
initial seek.

This fixes several seek problems, for example the one reported for the
FLAC decoder plugin:

 https://bugs.musicpd.org/view.php?id=4552

If there's a stream tag, don't let the song tag override it in the
next update_stream_tag() call.

Reduce runtime overhead.

Simply return an empty WritableBuffer, not a nulled one.

For type safety and code readability.