Fix incorrect value_type for concurrent_[unordered_]set::iterator

[kboyarinov]

Description

Fix an issue in concurrent_set and concurrent_unordered_set - iterator types should be constant for these classes.
Current implementation allows to write the following:

oneapi::tbb::concurrent_set<int> s = {1, 2, 3};
*s.begin() = 42;
// Now s is {42, 2, 3} - elements ordering is broken

The code above can brake elements ordering for ordered containers and place wrong elements into buckets for unordered.

Fixes # - issue number(s) if exists

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

Type of change

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• 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

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Other information

[pavelkumbrasev]

Please add comment why such approach is used.