Add NoThreadClone#isShareable method so the elements can dynamically decide if they need cloning or not

[vlsi]

Motivation and Context

Previously, components could implements NoThreadClone to prevent cloning in each thread, however, there was no way to "unimplement" the interface.

By default, isShareable returns true, so existing components work as before.

This improvement is needed for:

• Bug 66269: Add NoThreadClone to HeaderManager to reduce heap utilization #727
• Timers often have features like "global timer" vs "thread-local timer". Typically, timers have Map<ThreadGroup, ...>, however, making timer clonable or non-clonable depending on "local vs global" property might be a nicer way to code timers.

[vlsi]

It looks like another API is needed so the component can figure out if the actual element has been cloned or not.

For instance, in case like #727, HeaderManager#assertMutable() should be able to tell if the specific HeaderManager is cloned or not.
If a code creates a temporary HeaderManager object for the purpose of merging the headers, then the temporary HeaderManager should not fail with something like "oh, this is a shared instance, stop modifying it".

It sounds like NoThreadClone#freeze() method which would signal the component the instance is reused.

Then TreeCloner would call freeze on non-cloned objects.

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Unfortunately, it reminds me of the new memory management in Kotlin Native : https://kotlinlang.org/docs/native-migration-guide.html
Initially, they had a memory model which the user must call freeze object before sharing the object cross-threads.
That memory model has proven to be hard to use, and they went with a garbage collector.
Even though our goals are slightly different, it sounds related.