Bokeh protocol improvements

[philippjfr]

Three small fixes to improve event handling with bokeh protocol:

• Place hold on Document after adding root (avoid RootAdded events)
• Avoid sending updates without events entirely
• Only update colormapper if attributes have changed (avoid ModelChanged events and glyph redraws)

[philippjfr]

Ready to merge. At some point we should go through the entire bokeh backend and ensure that we don't needlessly set bokeh model attributes, the colormapper was just a very obvious case to do this since every change to it triggers a full rerender of the glyph.

[jlstevens]

Looks good! Merging.