Large speed up and benchmark for graphs with many points

[vii]

Adding fields to a structure after it is created can be slow on some
browsers, like Chrome.

Adding a demo to benchmark many points, this seems to speed up my
Chromium from about 2.8-3s to about 1.8s.

Thanks to Christopher Palmer (@thecav) for the analysis and specific
suggestion.

[coveralls]

Coverage decreased (-0.02%) to 90.174% when pulling eec9dae on vii:master into 8dc4700 on danvk:master.

[vii]

@danvk would love some feedback! @saboya has some improvements upon the optimizations

[danvk]

Thanks for this change @vii. I can confirm that it is also a speedup for me in Chrome 62.

I don't think a performance benchmark is appropriate for the gallery, so I'm going to revert that change. Have you seen tests/dygraph-many-points-benchmark.html? It does something similar.

[danvk]

I'll hold off on cutting a new release until @saboya sends a PR.

[saboya]

Hello.

The link that I made on my commit was actually just a reference, it's not an improvement over @vii 's code, it's just an implementation using a custom dataHandler on my fork of react-dygraphs. Since dygraphs releases usually take some time, I just did that in order to benefit from it right now.

Thanks for your work on dygraphs, and happy new years everyone :)