run_id added to INFO output (backported from 2.6).

This change is required to make Redis 2.4 compatible with Sentinel.

The Run ID is a field that identifies a single execution of the Redis
server. It can be useful for many purposes as it makes easy to detect if
the instance we are talking about is the same, or if it is a different
one or was rebooted. An application of run_id will be in the partial
synchronization of replication, where a slave may request a partial sync
from a given offset only if it is talking with the same master. Another
application is in failover and monitoring scripts.

src/redis.h

@@ -61,6 +61,8 @@
 #define REDIS_REPL_TIMEOUT 60
 #define REDIS_REPL_PING_SLAVE_PERIOD 10
 
+#define REDIS_RUN_ID_SIZE 40
+
 /* Hash table parameters */
 #define REDIS_HT_MINFILL        10      /* Minimal hash table fill 10% */
 
@@ -408,6 +410,7 @@ struct redisServer {
     char neterr[ANET_ERR_LEN];
     aeEventLoop *el;
     int cronloops;              /* number of times the cron function run */
+    char runid[REDIS_RUN_ID_SIZE+1];  /* ID always different at every exec. */
     time_t lastsave;                /* Unix time of last save succeeede */
     /* Fields used only for stats */
     time_t stat_starttime;          /* server start time */