Found with modernize-use-nodiscard
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Eliminates lots of overhead.

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with performance-move-const-arg
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-equals-default
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with performance-unnecessary-value-param
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

This function implements the "group" feature on top of the
Database::Visit() method.

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.

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

That way, each plugin can decide to implement it better.

Prepare to allow more complex expressions.

This enforces the "not nullptr" rule.

The duration of a song can have fractions of seconds
(quote from http://www.upnp.org/schemas/av/didl-lite-v2.xsd):

    The format of the duration string is:
    H+:MM:SS[.F+], or H+:MM:SS[.F0/F1]
    Where:
    +H              one or more digits to indicate elapsed hours,
    MM              exactly 2 digits to indicate minutes (00 to 59),
    SS              exactly 2 digits to indicate seconds (00 to 59),
    F+              any number of digits (including no digits) to indicate fractions of seconds,
    F0/F1   a fraction, with F0 and F1 at least one digit long,
                    and F0 < F1.
    The string may be preceded by an optional + or - sign, and the
    decimal point itself may be omitted if there are no fractional  seconds digits.

Until now, a duration with fractions of seconds could not be parsed and
resulted in an unknown duration. Only durations in the format "H+:MM:SS"
were feasible. This commit enables to read durations in the first format,
i.e. "H+:MM:SS[.F+]"

This is not necessary, because a strlen() on a literal gets optimized
away by the compiler.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.