Improved bokeh server functionality

[philippjfr]

• Allows starting bokeh server on a thread
• Execute bokeh server events eagerly if possible

Fixes #138
Fixes #181

[codecov-io]

Codecov Report

Merging #257 into master will decrease coverage by 0.41%.
The diff coverage is 32.46%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   89.32%   88.91%   -0.42%     
==========================================
  Files          44       44              
  Lines        5445     5492      +47     
==========================================
+ Hits         4864     4883      +19     
- Misses        581      609      +28

Impacted Files	Coverage Δ
panel/viewable.py	70.73% <18.91%> (-1.8%)	⬇️
panel/pane.py	81.99% <33.33%> (-0.41%)	⬇️
panel/layout.py	94.82% <33.33%> (-0.81%)	⬇️
panel/widgets.py	94.11% <33.33%> (-0.31%)	⬇️
panel/holoviews.py	82.67% <33.33%> (-0.75%)	⬇️
panel/util.py	78.64% <35.29%> (-3.9%)	⬇️
panel/__init__.py	56% <72.72%> (+3.82%)	⬆️

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