Convert thread stop handling into a publish/subscribe model

Convert thread stop handling into a publish/subscribe model

In later commits this will allow providers to subscribe to thread stop
events. We will need this in the FIPS module. We also make thread stop
handling OPENSSL_CTX aware (different OPENSSL_CTXs may have different
thread local data that needs cleaning up).

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9040)