Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

For exception-safety.

Reduce bloat.

Reduce bloat.  We never used the set lookup.

Make this non-fatal.  Just stop reading more objects.

Passing this to csvToStrings() will do the same.

Fixes crash when there's no SearchCaps element.

Eliminate the std::string bloat.

Delete the object when closing the database.

[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]