The main EventLoop can block for a long time while a client's command
runs, and is therefore inappropriate for internal engine I/O.  This
fixes a serious regression for at least the "httpd" output, which used
to be hard-coded for the IOThread, but now receives the main EventLoop
as an initialization parameter.

For the mixers, this doesn't make much of a difference.  They are not
latency critical.

Eliminate dependency on io_thread_get().

Forward the configured ReplayGainMode to all outputs and decoders.

Yet another global variable removed.

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

Move code from replay_gain_get_real_mode().

Use MaskMonitor to eliminate duplicate code.

Move the GlobalEvents::Monitor instance into class Instance.
Eliminate all global variables.