Ensure callbacks use pn.state.curdoc

[philippjfr]

Using bokeh's curdoc() function means the periodic callbacks cannot easily be used with pn.serve.

[codecov]

Codecov Report

Merging #1503 into master will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   86.26%   86.26%           
=======================================
  Files         142      142           
  Lines       15772    15772           
=======================================
  Hits        13606    13606           
  Misses       2166     2166           

Impacted Files	Coverage Δ
panel/callbacks.py	30.43% <33.33%> (ø)

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