This enforces the "not nullptr" rule.

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

Use milliseconds precision to reduce rounding errors.

Enables using range-based "for".

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

Replaces GLib's GError.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.