save peak memory usage as statistic and show it in INFO. Also a new INFO field was added showing the fragmentation ratio using the peak memory info.

src/redis.h

@@ -518,6 +518,7 @@ struct redisServer {
     long long stat_evictedkeys;     /* number of evicted keys (maxmemory) */
     long long stat_keyspace_hits;   /* number of successful lookups of keys */
     long long stat_keyspace_misses; /* number of failed lookups of keys */
+    size_t stat_peak_memory;        /* max used memory record */
     /* Configuration */
     int verbosity;
     int maxidletime;