aio_cxx: Fix mutual deadlock in PoolEIOflag test

aio_cxx: Fix mutual deadlock in PoolEIOflag test

The LibRadosAio.PoolEIOFlag test was unstable, it was hanging due to:

 - Deadlock: The main thread held the shared mutex ('my_lock') while
   calling thread join. This created a mutual deadlock.
    * Fix: Mutex is unlocked before thread join using RAII scopes.
 - also convert to std::jthread and drop the join

Fixes: https://tracker.ceph.com/issues/73946
Signed-off-by: Nitzan Mordechai <nmordec@ibm.com>