Add more quicklist info to DEBUG OBJECT

Adds: ql_compressed (boolean, 1 if compression enabled for list, 0
otherwise)
Adds: ql_uncompressed_size (actual uncompressed size of all quicklistNodes)
Adds: ql_ziplist_max (quicklist max ziplist fill factor)

Compression ratio of the list is then ql_uncompressed_size / serializedlength

We report ql_uncompressed_size for all quicklists because serializedlength
is a _compressed_ representation anyway.

Sample output from a large list:
127.0.0.1:6379> llen abc
(integer) 38370061
127.0.0.1:6379> debug object abc
Value at:0x7ff97b51d140 refcount:1 encoding:quicklist serializedlength:19878335 lru:9718164 lru_seconds_idle:5 ql_nodes:21945 ql_avg_node:1748.46 ql_ziplist_max:-2 ql_compressed:0 ql_uncompressed_size:1643187761
(1.36s)

The 1.36s result time is because rdbSavedObjectLen() is serializing the
object, not because of any new stats reporting.

If we run DEBUG OBJECT on a compressed list, DEBUG OBJECT takes almost *zero*
time because rdbSavedObjectLen() reuses already-compressed ziplists:
127.0.0.1:6379> debug object abc
Value at:0x7fe5c5800040 refcount:1 encoding:quicklist serializedlength:19878335 lru:9718109 lru_seconds_idle:5 ql_nodes:21945 ql_avg_node:1748.46 ql_ziplist_max:-2 ql_compressed:1 ql_uncompressed_size:1643187761

Author: mattsta

src/debug.c

@@ -306,11 +306,32 @@ void debugCommand(redisClient *c) {
             char *nextra = extra;
             int remaining = sizeof(extra);
             quicklist *ql = val->ptr;
-            double avg = (double)ql->count/ql->len;
+            /* Add number of quicklist nodes */
             int used = snprintf(nextra, remaining, " ql_nodes:%u", ql->len);
             nextra += used;
             remaining -= used;
-            snprintf(nextra, remaining, " ql_avg_node:%.2f", avg);
+            /* Add average quicklist fill factor */
+            double avg = (double)ql->count/ql->len;
+            used = snprintf(nextra, remaining, " ql_avg_node:%.2f", avg);
+            nextra += used;
+            remaining -= used;
+            /* Add quicklist fill level / max ziplist size */
+            used = snprintf(nextra, remaining, " ql_ziplist_max:%d", ql->fill);
+            nextra += used;
+            remaining -= used;
+            /* Add isCompressed? */
+            int compressed = ql->compress != 0;
+            used = snprintf(nextra, remaining, " ql_compressed:%d", compressed);
+            nextra += used;
+            remaining -= used;
+            /* Add total uncompressed size */
+            unsigned long sz = 0;
+            for (quicklistNode *node = ql->head; node; node = node->next) {
+                sz += node->sz;
+            }
+            used = snprintf(nextra, remaining, " ql_uncompressed_size:%lu", sz);
+            nextra += used;
+            remaining -= used;
         }
 
         addReplyStatusFormat(c,