By accident, this declared a global variable instead of adding the
"packed" attribute.

Explicit attribute initialization.

The MPD core takes care for checking the scheme.

[mk: modified to use MultiSocketMonitor instead of SocketMonitor]

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Debug-mode workaround for bogus assertion failure.

Add a flag that indicates whether the EventLoop is currently "busy".
As long as that flag is set, it does not need to be woken up - we can
simply add the DeferredMonitor to the list, and it will be caught by
EventLoop very soon.  This eliminates nearly all of the
DeferredMonitor overhead when compared to IdleMonitor, rendering
IdleMonitor mostly obsolete.

Reduce DeferredMonitor overhead.

Improved support for added events.

This call is not thread-safe.

Client::Close() installs a TimeoutMonitor, which is not something we
should do during shutdown.