Ensure cleanup happens when enabling --warm or --autoreload with Bokeh 2.4

[philippjfr]

Fixes #2755

[codecov]

Codecov Report

Merging #2760 (b3cf204) into master (dd8aeca) will increase coverage by 0.61%.
The diff coverage is 86.20%.

@@            Coverage Diff             @@
##           master    #2760      +/-   ##
==========================================
+ Coverage   82.03%   82.65%   +0.61%     
==========================================
  Files         187      188       +1     
  Lines       23963    24049      +86     
==========================================
+ Hits        19658    19877     +219     
+ Misses       4305     4172     -133     

Impacted Files	Coverage Δ
panel/tests/command/test_serve.py	85.00% <85.00%> (ø)
panel/command/serve.py	39.56% <100.00%> (+22.52%)	⬆️
panel/tests/conftest.py	95.90% <100.00%> (+0.21%)	⬆️
panel/pane/holoviews.py	79.65% <0.00%> (+0.21%)	⬆️
panel/pane/vtk/vtk.py	96.83% <0.00%> (+0.45%)	⬆️
panel/pane/plot.py	59.69% <0.00%> (+0.51%)	⬆️
panel/pane/vega.py	82.64% <0.00%> (+0.82%)	⬆️
panel/io/callbacks.py	68.00% <0.00%> (+1.00%)	⬆️
panel/viewable.py	70.44% <0.00%> (+2.06%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dd8aeca...b3cf204. Read the comment docs.

[hoxbro]

I'm seeing some (new) problems with this fix.

I have this code

import panel as pn
pn.panel("Test").servable()

Which I run with panel serve race_condition.py --autoreload, but when I reload the page multiple times I get this error:

2021-09-20 22:06:31,569 DocumentLifeCycleHandler on_session_destroyed callback <bound method PeriodicCallback._cleanup of PeriodicCallback(callback=functools.partial(<function _reload_on_update at 0x00000250A9488AF8>, {'race_condition.py': 1632167947.1386704}), count=None, name='PeriodicCallback00196', period=500, running=False, timeout=None)> failed with following error: 'DocumentCallbackManager' object has no attribute '_change_callbacks'

Screenshot

Screencast

Video.mp4