storage: Switch to new def on pool-destroy

storage: Switch to new def on pool-destroy

Currently, we share the idea of old & new def with domains. Users can
*-edit an object (domain, pool) which spawns a new internal
representation for them. This is referenced via
{domainObj,poolObj}->newDef [compared to ->def]. However, for pool we
were never overwriting def with newDef. This must be done on
pool-destroy (like we do analogically in domain detroy).