Was broken by commit 8a86460b

Closes #87

We will reuse these tags elsewhere.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

By declaring the variable-length array to have a nominal size of 1,
struct TagPoolSlot shrinks from 24 bytes to 16 bytes, because "ref"
and "item" now both fit in one machine word.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

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.

Fixes deadlock.