Add tsan_decr() API, counterpart of tsan_counter()

The existing tsan_counter() API increments a reference counter.
Provide a new API, tsan_decr(), to decrement such a reference counter.
This can be used, for example, when a reference is added to the session_ctx's
sess_accept stats but should more properly be tracked in the regular ctx's
statistics.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7464)