Make ClientSession slot-based class

[asvetlov]

Convert debug-mode warning about a wild session modification into a strict error.

[codecov-io]

Codecov Report

Merging #3463 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3463   +/-   ##
=======================================
  Coverage   97.95%   97.95%           
=======================================
  Files          44       44           
  Lines        8558     8558           
  Branches     1383     1383           
=======================================
  Hits         8383     8383           
  Misses         72       72           
  Partials      103      103

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