Minor rework of SpinLock class for making it more efficient on Unlock()

[noSTALKER]

• atomic_flag is initialized using the recommended ATOMIC_FLAG_INIT
• Unlock() function clears the atomic_flag using more efficient std::memory_order_acquire
• Unlock() and Lock() methods are now both noexcept
• Removed unnecessary include files

[msftclas]

All CLA requirements met.

[noSTALKER]

Just a query for future. I haven't seen noexcept keyword usage much in the code base. Is it intentional or i can include it in future pull request?

[noSTALKER]

Also the the functions are name Lock() and UnLock() so it can't be used in a code section safely which can throw exception with std::scoped_lock() or std::lock_guard() as it requires function names to be lock() and unlock()

[AlphardWang]

Just a query for future. I haven't seen noexcept keyword usage much in the code base. Is it intentional or i can include it in future pull request?

Currently we don't have intention to use or not to use noexcept. Please feel free to use it in your PR. We will do refactoring and add it into readme if it becomes an explicit requirement in the future.

[AlphardWang]

Personally I don't recommend to use default member initializer since it may become confused and hard to read when there are multiple data members or constructors. But it should be fine here since it is the only data member and all implementation is in same file now.

[noSTALKER]

This is the recommended way of in initializing std::atomic_flag, that is why i didn't used it in initializer list as it requires copy initialization using the ATOMIC_FLAG_INIT

[AlphardWang]

Please replace the tabs with spaces.