Replace simulation thread with simulation task in BatchContext

[jonathan-eq]

Issue
Continuation of #8466

Creating scheduler message queues (via asyncio.Queue) is done in different asyncio loop (and thread too), which might cause an issue when running in python3.8. This replaces the sim thread with sim task in order to make sure that we run on the same loop.

Approach

Replace & simplify.

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.89%. Comparing base (22e95fc) to head (dd263fe).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/simulator/batch_simulator_context.py	91.66%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8681   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files         342      342           
  Lines       21213    21207    -6     
=======================================
- Hits        19281    19276    -5     
+ Misses       1932     1931    -1     

Flag	Coverage Δ
gui-tests	76.10% <13.88%> (-0.04%)	⬇️
integration-tests	53.37% <13.88%> (-0.04%)	⬇️
unit-tests	68.64% <91.66%> (+<0.01%)	⬆️

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[jonathan-eq]

Closing this as it requires more changes in everest. See additional comments in #8466