Handle duplicate attempts to refresh auth tokens

[philippjfr]

When an auth token refresh is in process we should not attempt another refresh at the same time, so we detect the case where the refresh is in flight and simply await it by sleeping until the refreshed tokens are available.

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (3a3b1be) 84.17% compared to head (bde5e56) 22.30%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
panel/io/notebook.py	30.76%	9 Missing ⚠️
panel/auth.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6233       +/-   ##
===========================================
- Coverage   84.17%   22.30%   -61.88%     
===========================================
  Files         301      301               
  Lines       45099    45114       +15     
===========================================
- Hits        37963    10061    -27902     
- Misses       7136    35053    +27917     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	22.30% <21.05%> (-49.55%)	⬇️

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