Do not push a new scope on StartSpan

[cleptric]

This partially reverts #886 and fixes #1009.

Pushing a new scope on each new span has too many side effects, like making it impossible to access the "root" scope to attach certain attributes like the user to it.
Instead, we now only set the span on the scope when it's started and set it back to its parent when it finishes.
As it's a hard requirement to always clone the hub when working with go routines, we now have a simulated isolation scope.

Parent child relations are correctly maintained with this changes, as well as error -> span correlation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.08%. Comparing base (523e1b4) to head (7f189ee).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
- Coverage   85.12%   85.08%   -0.05%     
==========================================
  Files          53       53              
  Lines        5276     5275       -1     
==========================================
- Hits         4491     4488       -3     
- Misses        640      642       +2     
  Partials      145      145              

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[cleptric]

@Darevski just pinging you here as you opened the PR with the initial changes last year. In my testing, all the issues you reported back then are still fixed with these changes.