Remove malloc failure checks

We trust zmalloc to kill the whole process on memory failure
Add quicklist info to DEBUG OBJECT

Added field 'ql_nodes' and 'ql_avg_per_node'.

ql_nodes is the number of quicklist nodes in the quicklist.
ql_avg_node is the average fill level in each quicklist node. (LLEN / QL_NODES)

Sample output:
127.0.0.1:6379> DEBUG object b
Value at:0x7fa42bf2fed0 refcount:1 encoding:quicklist serializedlength:18489 lru:8983768 lru_seconds_idle:3 ql_nodes:430
ql_avg_per_node:511.73
127.0.0.1:6379> llen b
(integer) 220044