Fix constraint arena construction

[pavelkumbrasev]

Description

Currently constrained arena might be initialized (first touch to fields) on inappropriate constraints because initializing thread doesn't apply constraints during initialization.
This PR reuses binding_observer to apply constraints on thread that performs initialization.

Fixes # - issue number(s) if exists

• - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

Type of change

Choose one or multiple, leave empty if none of the other choices apply

Add a respective label(s) to PR if you have permissions

• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

• Yes
• No
• Unknown

Notify the following users

List users with @ to send notifications

Other information

[aleksei-fedotov]

Alternative approach would be deferring initialization of the fields until the task is spawned in the arena or the thread is joined the arena for task execution, essentially adding more laziness to the process. In this case, we can save on superfluous calls to enter and exit observer. It will even save on arena initialization if the task will not be spawned and thread would not be entering arena for execution at all.

Have this approach been considered?

[pavelkumbrasev]

Alternative approach would be deferring initialization of the fields until the task is spawned in the arena or the thread is joined the arena for task execution, essentially adding more laziness to the process. In this case, we can save on superfluous calls to enter and exit observer. It will even save on arena initialization if the task will not be spawned and thread would not be entering arena for execution at all.

This is an interesting suggestion. I believe it will require extra synchronization on each arena entry (we already have one for task_arena::initialize) so I'm not sure if it will be effective.
Added TODO comment.

[isaevil]

LGTM