OSDMonitor::_check_become_tier cannot prevent client from setting multiple tiers in a particular way
suppose there are three pools,A B C.
if we add B to A as a tier,then add C to B as a tier,
the current implement will return with EINVAL to prevent setting multiple tiers.
but we can success setting multiple tiers in reverse order,that is,add C to B first,then add B to A as a tier,
_checkout_become_tier will not prevent it, and it might cause many serious problems, like risk of correctness.