Fix race condition in autoreload

[philippjfr]

When autoreload detects a modified file it clears out the sys.modules and then reloads just the session that detected the changed file. Subsequent sessions that try to detect the changed file will then no longer find the changed module in the sys.modules (meaning they can't detect a change), or they will find that the already restarted sessions has already repopulated sys.modules and delete that newly loaded module (which will potentially break the already restarted session). In the new setup whenever any session detects that a module was changed it immediately halts the watchers on other threads and then reloads all sessions.

This has the potential to fail for sessions that do not have a frontend connected (since the reload mechanism is triggered by refreshing the browser window) and those sessions will stick around. Someone could in theory reconnect to such an old session programmatically but since autoreload is meant for interactive use this does not seem like a big worry.

Fixes #2531

[codecov]

Codecov Report

Merging #2539 (a384125) into master (4473ab2) will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master    #2539   +/-   ##
=======================================
  Coverage   83.20%   83.20%           
=======================================
  Files         184      184           
  Lines       23236    23236           
=======================================
  Hits        19334    19334           
  Misses       3902     3902           

Impacted Files	Coverage Δ
panel/io/reload.py	65.90% <83.33%> (-0.76%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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