Callbacks enhancements and docs

[philippjfr]

Closes #1447

• Allows using pn.serve to start a server on a StoppableThread
• Adds a pn.state.add_periodic_callback method which will eventually replace the method on each object
• Adds docs about modifying Bokeh models on threads and scheduling periodic callbacks

[codecov]

Codecov Report

Merging #1507 into master will increase coverage by 0.00%.
The diff coverage is 44.44%.

@@           Coverage Diff           @@
##           master    #1507   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files         142      143    +1     
  Lines       15779    15786    +7     
=======================================
+ Hits        13613    13620    +7     
  Misses       2166     2166           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
panel/io/callbacks.py	31.11% <31.11%> (ø)
panel/io/state.py	57.26% <31.25%> (-2.37%)	⬇️
panel/reactive.py	85.30% <50.00%> (-0.35%)	⬇️
panel/viewable.py	70.79% <50.00%> (+1.56%)	⬆️
panel/callbacks.py	100.00% <100.00%> (+69.56%)	⬆️
panel/io/__init__.py	100.00% <100.00%> (ø)
panel/io/server.py	66.51% <100.00%> (+1.09%)	⬆️
panel/tests/conftest.py	94.68% <100.00%> (ø)
panel/tests/test_server.py	100.00% <100.00%> (ø)
... and 1 more

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