Unlock Document and dispatch events if we have lock

[philippjfr]

This PR ensures that when even on the server events are dispatched immediately. This is achieved by adding an unlock context manager which dispatches all ModelChangedEvents triggered in the context body to all sockets on current sessions. I have to do significantly more testing to figure out if this is actually safe.

Fixes #778

[codecov]

Codecov Report

Merging #857 into master will decrease coverage by 0.17%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #857      +/-   ##
==========================================
- Coverage   69.94%   69.76%   -0.18%     
==========================================
  Files         104      104              
  Lines       10463    10499      +36     
==========================================
+ Hits         7318     7325       +7     
- Misses       3145     3174      +29

Impacted Files	Coverage Δ
panel/widgets/base.py	36.06% <0%> (-0.61%)	⬇️
panel/io/__init__.py	0% <0%> (ø)	⬆️
panel/io/server.py	17.27% <12.5%> (-1.96%)	⬇️
panel/pane/holoviews.py	75.25% <66.66%> (+0.06%)	⬆️
panel/viewable.py	56.28% <66.66%> (+0.13%)	⬆️
panel/pane/base.py	59.47% <66.66%> (+0.26%)	⬆️

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[philippjfr]

Going to merge and make sure I test it extensively before release.