Do some more cleanup in the RCU code

Do some more cleanup in the RCU code

Only a minimum of 2 qp's are necessary: one for the readers,
and at least one that writers can wait on for retirement.
There is no need for one additional qp that is always unused.
Also only one ACQUIRE barrier is necessary in get_hold_current_qp,
so the ATOMIC_LOAD of the reader_idx can be changed to RELAXED.
And finally clarify some comments.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27012)