Refactor init_get_thread_local to be more understandable

Refactor init_get_thread_local to be more understandable

We currently have a single function that does thread_local key
allocation/cleanup/fetching for our OSSL_init_thread_start/stop apis,
and its pretty confusing.  Wrap it up in some helper functions to make
it more clear at the call sites what we're trying to do.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)