-* Use the same pointer of db->dict in db->expire hash table for keys.
- 1) Set the keyptr hash table type key destructor to NULL.
- 2) Don't copy the key in setExpire(), but instead lookup the same key
- in the dict hash table, and use it.
- 3) Make sure (and add comments about this) that when a key is deleted or
- an expire is touched, the order is: delete the expire, delete the key.
- 4) Make sure the SETEX command works well in all the cases. Add tests.