title: Displaying only one tooltip when using the HoverTool() tool
body: <p>I am plotting very many points in Bokeh, and I have added the <a href="http://bokeh.pydata.org/en/0.11.1/docs/reference/models/tools.html#bokeh.models.tools.HoverTool" rel="noreferrer">HoverTool</a> to the list of tools of the figure, so that the mouse shows the <code>x,y</code> coordinates of the mouse when close to a glyph.</p>
  
  <p>When the mouse gets close to a set of glyphs closely packed together, I get as many tooltips as glyphs. I want instead only one tooltip, the one of the closest glyph. This isn't just a presentation detail, because for very many points this results:</p>
  
  <ul>
  <li>in slow interaction with the plot, with the browser getting stuck while all tooltips are generated</li>
  <li>in a very long tooltip, where the same information is repeated as many times as many glyphs are close to the cursor</li>
  </ul>
  
  <p>An example follows, with the code to replicate the behaviour:
  <a href="https://i.stack.imgur.com/Yb6A8.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Yb6A8.png" alt="enter image description here"></a></p>
  
  <pre><code>import numpy.random
  from bokeh.plotting import figure, output_notebook, show
  from bokeh.models import HoverTool
  output_notebook()
  
  hover = HoverTool()
  hover.tooltips = [("(x,y)", "($x, $y)")]
  
  x = numpy.random.randn(500)
  y = numpy.random.randn(500)
  
  p = figure(tools=[hover])
  p.circle(x,y, color='red', size=14, alpha=0.4)
  
  show(p)
  </code></pre>