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.

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.

The old check

 unsigned(value) > std::numeric_limits<unsigned>::max()

.. cannot ever fail.

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