Fixes part 4 of https://github.com/MusicPlayerDaemon/MPD/issues/1490

Fixes regression by commit 8e0d8109 which is 2 years old, and nobody
noticed.  D'oh, how embarassing!

SonarLint reports the latter to be better:

std::scoped_lock basically provides the same feature as std::lock_guard,
but is more generic: It can lock several mutexes at the same time, with a
deadlock prevention mechanism (see {rule:cpp:S5524}). The equivalent code
to perform simultaneous locking with std::lock_guard is significantly more
complex. Therefore, it is simpler to use std::scoped_lock all the time,
even when locking only one mutex (there will be no performance impact).
Signed-off-by: Rosen Penev <rosenp@gmail.com>

C arrays can be used with make_unique in C++17.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The latter is easier to read and is the "correct" thing to do.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This way, we save the overhead for acquiring the tag_pool_lock.

Also, use RemoveAll() instead of directly clearing TagBuilder::items in
most cases, as its elements represent references that must be released.

Closes #1023

This reverts commit 1532983f.  This
optimization was bad because now all strings match if they are a
prefix of another string, and this caused collisions in the tag string
pool, corrupting the database.

This commit adds some tags that are (mostly) interesting for listeners
of classical music.

Ensemble
--------
This is an ensemble that is playing the music, such as Wiener
Philharmoniker. The tag can be used to distinguish the ensemble from the
conductor, composer, soloist, and ensemble, that are generally all in
the "ARTIST" tag.

Movement
-------
The movement number and movement (name) of this track, i.e.  "II" and
"Allegro".

ComposerSort
------------
Allows us to look for Beethoven's 9th under B, for Beethoven, not L for
Ludwig.

Location
--------
This is the location of the recording, e.g. "Wiener Musikverein".

Shorter.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Conductor was incorrectly saved to Performer tag in MPD database