Modify ossl_init_thread_start api to support freeing thread data without per-key destructor

[nhorman]

#552 is meant to investigate manners in which ossl_init_thread_start deletes per thread data in an in-appropriate manner. This task tracks the implementation of the output of that task. To be updated when a consensus is reached there. Current proposal is:

• ossl_init_thread_start should be cloned to ossl_init_thread_start_ex to accept an additional parameter *data, which holds a pointer to thread local data - cloning is to preserve (allbeit broken) ABI for existing providers
• THREAD_EVENT_HANDLER should be expanded to hold a pointer to (1) above
• OSSL_thread_stop_handler_fn should accept a second parameter data to pass (1) to the callback
• init_thread_stop should call (3) with the extra parameter stored in (2)

[t8m]

Should this be transferred to openssl/project?

[nhorman]

It certainly could be, but I'm not sure I see a significant value in doing so, unless there just some book keeping value there

[mattcaswell]

It does not belong in this repo IMO and should be transferred