Optimize get_address_deferred_credits in pos_final_state.rs

[Leo-Besancon]

Currently, we iterate over all the DeferredCredits keys while we're only interested in a specific address.

We don't have to visit every keys. We might want instead to use a while loop, and query only the keys corresponding to the wanted address.

slot_serializer.serialize(cur_slot, &mut key)
addr_serializer.serialize(address, &mut key)
deferred_credit_prefix!(key)

In the case it's not found, go immediatly to the next existing slot key instead of iterating over all addresses.

Originally posted by @Leo-Besancon in #4152 (comment)