Investigate embedding bokeh protocol messages

[philippjfr]

In #1894 we added compatibility with the upcoming changes in bokeh 0.12.10 to send notebook events based on the bokeh protocol. This had the added benefit of giving us binary transfer but only works when sending the data live. For embedding data when rendering a HoloMap we still use our old code to generate JSON patches. It should be possible to use the protocol here as well but it requires some work rewriting both Python and JS components and add conditional logic to handle old versions of bokeh. Therefore I don't think this is of high priority, we should try to drop the old approach once bokeh 1.0 has been released.

[philippjfr]

This was merged in #1949.

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