Distinguish between session creation and execution in session info

[philippjfr]

Panel has supported recording information about user sessions for a while. In doing so it recorded three events, 'started', 'rendered' and 'ended'. However the 'started' event was really recording the time after the session had already been created and fully initialized. This adds a fourth key called 'launched', which records when the initial request arrives but before the dashboard script has been executed.

This means that we can separately compute execution time (server-side) and time to render since initial request (client-side).

[codecov]

Codecov Report

Merging #1909 (b551b24) into master (47256ca) will increase coverage by 0.07%.
The diff coverage is 87.80%.

@@            Coverage Diff             @@
##           master    #1909      +/-   ##
==========================================
+ Coverage   85.02%   85.10%   +0.07%     
==========================================
  Files         152      152              
  Lines       18883    18913      +30     
==========================================
+ Hits        16056    16096      +40     
+ Misses       2827     2817      -10     

Impacted Files	Coverage Δ
panel/io/state.py	56.21% <83.33%> (+4.68%)	⬆️
panel/io/server.py	65.71% <88.57%> (+2.96%)	⬆️
panel/io/rest.py	31.63% <0.00%> (+4.08%)	⬆️

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