Support dynamic thread initialization

[nickjong]

For the C++ bindings, it would be nice to avoid the overhead of repeatedly invoking WTF_THREAD_ENABLE in situations where the identity of individual threads is not directly exposed to the code being instrumented, or where client code otherwise has no practical way to ensure that WTF_THREAD_ENABLE is called only once per thread in some thread pool.

[vertexodessa]

it's not exactly what you requested but it might fit your needs #579