Made proxy work for multiple maps in Shiny

[hckiang]

This commit fixes this issue: https://github.com/crazycapivara/deckgl/issues/224

Apparently Shiny's addCustomMessageHandler is a misnomer: it should be called setCustomMessageHandler because each time you add a handler to the same keyword it'll replace the previous one. This fix just gives handlers of each deckgl widget a unique name so that they can coexist.