[ASDisplayNode] Implement a std::atomic-based flag system for superb performance

[appleguy]

Although Objective-C atomics are equally fast, or better that std::atomic when
access through method calls, for the most intense use cases it is best to avoid
method calls entirely.

In ASDisplayNode, we could benefit significantly from avoiding both method calls
(already true today) but also avoid locking the mutex, for both CPU and contention
gains.

There will still be many methods that need locking for transactional
consistency - however, there are currently dozens of accessor methods that could
avoid frequent lock / unlock cycles with use of atomics, and other usages of the
ivars directly where locking could be delayed until after early-return conditions
are checked and passed.

[ghost]

🚫 CI failed with log

[Adlai-Holler]

Nice! Left an idea to make it more scalable.

[Adlai-Holler]

Let's remove this field until we use it.

[Adlai-Holler]

Let's eliminate the flags argument from checkFlag and setFlag, and then remove the atomic_* macros.
Let's also have setFlag return the old value of the flag, so it would shake out like this:

#define checkFlag(flag) ((_atomicFlags.load() & flag) == flag)
// Returns the old value of the flag as a BOOL.
#define setFlag(flag, x) (((x ? _atomicFlags.fetch_or(flag) : _atomicFlags.fetch_and(~flag)) & flag) != 0)
 
// In code:
BOOL sync = checkFlag(ASDNFlagSynchronous);
setFlag(ASDNFlagSynchronous, YES);

[Adlai-Holler]

Alrighty! I'm down with this and we'll see how nice we can get the naming as we go.

[ghost]

	1 Warning
⚠️	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

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[appleguy]

Thanks for your help on this @Adlai-Holler !