Without the flush, ReadPage() may not return any data, or not all
data.  This may result in incomplete ddata the new "header" page,
corrupting streams with some encoders such as Vorbis.

Fixes #145

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

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.

Make it "const".

.. and move the plugins to src/encoder/plugins/.

This call is not thread-safe.

Fixes more thread-safety bugs.

Add a Page queue to class HttpdOutput, and use DeferredMonitor to
flush this queue inside the IOThread.  This fixes a thread-safety
issue: much of EventLoop is not thread-safe, and the httpd plugin
ignored that problem.