Replace system_clock::time_point with deterministic precision in public API

[mr-smidge]

Several points in the public API refer to a point in time using std::chrono::system_clock::time_point, e.g.

struct track_row
{
    ...
    std::chrono::system_clock::time_point date_added;
    ...
};

However, this mandates that the precision of the local system clock be used. In fact, whilst it is helpful to have the guarantee that the point-in-time is measured since the UNIX epoch (a guarantee made by system_clock since C++20), it is not helpful that the resolution is unknown.

Instead, an appropriate precision as required by the underlying database formats (and a sensible decision on precision taken for the high-level API) could be specified, e.g.

struct track_row
{
    ...
    std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds> date_added;
    ...
};

From C++20 onwards, there is an alias sys_seconds for the above.