Track actions performance

[zalmoxisus]

Similarly to timestamp property for each stored entry of actionsById, we're introducing perf property, so we'll be able to know not only when an action started, but also how much time it took to execute its reducer.

This should be rather useful for measuring the performance.

@gaearon, what do you think?

[gaearon]

Maybe timeSpentInReducer or duration?

[zalmoxisus]

duration is indeed better. I don't think it's necessary to clarify that it's about reducers. Similarly, timestamp is also when the reducer starts, not when the action creator is called.

[gaearon]

Fair enough. We just need to manage expectations because when people think of “how much time it took to process an action” they usually want to know how long components took to update, not just reducers.

[zalmoxisus]

It would be great to show perf not only for reducers, but it doesn't seem to be possible here.
Better to specify it in the docs, so I've updated the README, please amend it when you get time.