The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

Allows more forward declarations.

See commit 71f0ed8b

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

With this commit, multi-player support becomes possible... it's just
not wired to the frontend yet.

This is based on massive amounts of refactoring work I did over the
past 9 years.

Eliminate global variables.

Replaces the old BoundMethod template.

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

All remaining events are specific to the Partition.

Use MaskMonitor to eliminate duplicate code.

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

Keep only a reference.

Replaces global variable "main_loop".

Decouples db/update/Remove.cpp from global variables.

Move from db/DatabaseGlue.cxx, eliminating global variable.