Keep track of a list of currently-connected clients.

Use `socket.getpeername()` to get an identifier for each connection and
include this in each log message. This function is documented as not
being available on all systems, but it's unclear which systems this
involves.

Also log a message on client connect and disconnect events. If the
disconnection reason is because the client sent a blank line, match MPD
by returning a protocol error then hanging up. Escape curly braces.

Getting this command puts the connection into a special mode where it
awaits MPD events (like the player changing state or the playlist
changing due to other clients interacting with the server.

The MPD specification states that events should queue while a client is
connected, and when it issues the `idle` command any matching events
should be sent immediately if there are any, or as soon as they happen
otherwise.

A new `ControlConnection` is created each time a client connects over
a new control socket. This is used to forward events from the player,
and also for debugging utilities that are not part of the real MPD
protocol.

This new feature reuses as much infrastructure from the normal protocol
handling as possible (e.g. `Command` for parsing messages). While the
normal connection delegates to server `cmd_*` methods which are string
generators, the control connections delegate to `ctrl_*` methods defined
on the connection itself that are full coroutines.