Performance of SVG axes

[OliverForeman]

The performance of SVG axes (e.g. drawn through cartesianChart) is now becoming a bottleneck for the highest performing WebGL charts. Ideally we need a solution to help push the best performance.

1. Document ways to improve the performance of axes e.g. By recycling DOM Nodes
2. Create an axis implementation with better performance e.g. Using canvas

[adilparvez]

Testing the existing code with default/identity key for the dataJoin in axisBase shows there isn't much difference in performance, most of the time is spent setting text/attributes on the elements. Recycling DOM nodes might not make much of a difference then...