Improve map rendering with lot of markers

[BoboTiG]

This could be done in several steps:

1. Force the use of the preferCanvas option.
2. Check those options as well.
3. For the current day, display all markers. For previous days, print only the fist and last markers?
4. Remove marker shadow to ease rednering.

Let's benchmark those ideas and implement them if results are better.

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[BoboTiG]

An idea: http://leaflet.github.io/Leaflet.markercluster/