Replace calculations of record durations with new `Duration` method

[dominikbraun]

There are at least two places in the code where the duration of a record (e.g. 2h if the record started at 2PM and ended at 4PM) is calculated like so:

record.end.sub(record.start)

Those calculations should be replaced with a new Duration method:

record.Duration()

The implementation of that method would just be something like this:

func (r *Record) Duration() time.Duration {
    return r.end.sub(r.start)
}

[alksmt]

Hey, created a pull request for it including one more use case. Along with usage of record.end.sub(record.start) there were cases for records being tracked at the moment, this method will handle it too now.

[dominikbraun]

Rssolved by #139.