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.

Avoid wasting 4 kB stack per directory level.

There were two ways this could fail:

1. division by zero when sysconf(_SC_PAGESIZE)==0

2. mmap() failure because the size parameter is not aligned to page
   size

Neither ever happened: sysconf() never fails, and the only caller
passes a size that is already aligned.  Phew.