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.

Do all I/O in the IOThread and not in the main thread.  This solves an
upcoming deadlock problem.

Don't let the server care for client problems.