Now all the commands returning a multi bulk reply against non existing keys will return an empty multi bulk, not a nil one

use directly the real key object in VM I/O jobs to match by pointer, and to handle different keys with the same name living in different DBs, but being at the same moment in the IO job queues

Prevent hash table resize while there are active child processes in order to play well with copy on write

free hash table entries about no longer active classes, so that PUBSUB can be abused with millions of different classes