Improve onEnd details

[vitaly-t]

Operator onEnd can be improved with extra details about duration, by providing min/max duration, along with indexes of where those were reached.

This is to help locate deviations in iterable performance.

For example, have duration inside IIterationSummary of the following type:

export interface IDuration {
    average: number; // averge duration
    max?: {delay, index, value} // delay = waiting time, index when the value was reached, the value
    min?: {delay, index, value}; // same as above
    total: number; // total duration
}

This would be a breaking change, because duration property changes.

[vitaly-t]

This change now has been merged into master.

[vitaly-t]

Updated onEnd released in v2.0.0-beta.7