Must lock the mutex before calling any of the unprotected InputStream methods.

Closes #462

Fixes build failure after commit
0cea67ee

GCC 9's libstdc++ is unable to use forward-declared types as
std::vector item because the compiler wants to resolve `noexcept()` on
the item destructor.

libcdio_paranoia was split from libcdio in version 90, and at the same
time, the header was moved from cdio/paranoia.h to
cdio/paranoia/paranoia.h.  We can easily detect this version at
compile time which is faster than configure time.

Re-add build support for the fixed-point Vorbis (Tremor) decoder, which
was dropped when switching from Autotools to Meson.

Note, that it is not possible to build both, the Vorbis and the Tremor
decoder.

Closes: #405

Closes https://github.com/MusicPlayerDaemon/MPD/issues/456

Similar to 5a5229b4: use more C++14
constexpr.

"De-facto" field mappings are available at http://wiki.hydrogenaud.io/index.php?title=Tag_Mapping.

Closes #343

fails on it. This fixes.