src: improve thread safety of TaskQueue

[codebytere]

Initial effort to fix #56236.

Improve thread safety of TaskQueue by making locking of TaskQueue explicit and thus hopefully preventing a crash due to uv_async_send being called with nullptr.

Please let me know if i headed in the wrong direction with this and i'll rework it!

[codecov]

Codecov Report

Attention: Patch coverage is 97.80220% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.25%. Comparing base (5077ea4) to head (f1b6765).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_platform.cc	97.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #57910    +/-   ##
========================================
  Coverage   90.25%   90.25%            
========================================
  Files         630      630            
  Lines      185691   185944   +253     
  Branches    36407    36443    +36     
========================================
+ Hits       167589   167827   +238     
- Misses      10994    11012    +18     
+ Partials     7108     7105     -3     

Files with missing lines	Coverage Δ
src/node_platform.h	80.00% <100.00%> (+5.00%)	⬆️
src/node_platform.cc	88.75% <97.77%> (+0.89%)	⬆️

... and 50 files with indirect coverage changes

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[mcollina]

@codebytere I wonder if this could also help with #54918.

[bnoordhuis]

LGTM with some suggestions.

[bnoordhuis]

Suggested change

	std::vector<std::unique_ptr<Task>> tasks_to_run;
	{
	auto locked = scheduler->tasks_.Lock();
	std::unique_ptr<Task> task;
	while ((task = locked.Pop())) {
	tasks_to_run.push_back(std::move(task));
	}
	}
	auto tasks_to_run = scheduler->tasks_.Lock().PopAll();

That's equivalent, right?

[bnoordhuis]

Same as above:

Suggested change

	std::vector<std::unique_ptr<DelayedTask>> delayed_tasks_to_schedule;
	{
	auto locked_tasks = foreground_delayed_tasks_.Lock();
	std::unique_ptr<DelayedTask> delayed;
	while ((delayed = locked_tasks.Pop())) {
	did_work = true;
	delayed_tasks_to_schedule.push_back(std::move(delayed));
	}
	}
	for (auto& delayed : delayed_tasks_to_schedule) {
	auto delayed_tasks_to_schedule = foreground_delayed_tasks_.Lock().PopAll();
	for (auto& delayed : delayed_tasks_to_schedule) {
	did_work = true;

[bnoordhuis]

Can you add a comment to their definitions in node_platform.h explaining the locks should always be acquired in this order?