Use this function instead of CheckError() when we already know an
error has occurred.

Obsoleted by commit d4d4d621

release v0.19.14

Fixes regression by commit 42f7df96

http://bugs.musicpd.org/view.php?id=4470

This reduces overhead for two reasons:

1. calls to Remove() are non-blocking

2. RunDeferred() may work on large chunks at a time, reducing the
   number of RunDeferred() calls

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

When a reference counter is at its limit, don't allocate a new
TagPoolSlot - that would result in many TagPoolSlot instances with
ref==1.  This in turn would make the linked list very very large,
which means quadratic runtime for many operations.

All remaining events are specific to the Partition.

Use MaskMonitor to eliminate duplicate code.