Avoid tornado import

[philippjfr]

Replaces tornado.gen decorator with async method.

[codecov-commenter]

Codecov Report

Merging #5321 (c625aa0) into master (c265cc1) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5321      +/-   ##
==========================================
- Coverage   86.74%   86.74%   -0.01%     
==========================================
  Files         299      299              
  Lines       62218    62215       -3     
==========================================
- Hits        53969    53966       -3     
  Misses       8249     8249              

Impacted Files	Coverage Δ
holoviews/plotting/bokeh/callbacks.py	59.60% <100.00%> (-0.15%)	⬇️

---

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 c265cc1...c625aa0. Read the comment docs.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.