Improve performance for addEventKeys

[MxFr]

I improved the performance of my charts with large datasets significantly by changing the getEventKey function in away that a new object is only created when eventKey is set in the props.

My reasoning for this change is:

1. If the datum has already an eventKey field, assign adds nothing.
2. If the index is used as fallback for eventKey the fallback can be (and already is in most cases) performed in the Components getBaseProps.
3. If the eventKey is computed the old behavior is required.

In my use case the perfomance of chart updates nearly doubled.

[boygirl]

@MxFr Thanks so much! That's an exciting perf improvement. From my first look this looks good, but I will give it a more thorough review tonight / tomorrow.

[boygirl]

@MxFr This looks good. Merging now.