Sentinel: Redis-side support for slave priority.

A Redis slave can now be configured with a priority, that is an integer
number that is shown in INFO output and can be get and set using the
redis.conf file or the CONFIG GET/SET command.

This field is used by Sentinel during slave election. A slave with lower
priority is preferred. A slave with priority zero is never elected (and
is considered to be impossible to elect even if it is the only slave
available).

A next commit will add support in the Sentinel side as well.

src/redis.h

@@ -62,6 +62,7 @@
 #define REDIS_REPL_PING_SLAVE_PERIOD 10
 
 #define REDIS_RUN_ID_SIZE 40
+#define REDIS_DEFAULT_SLAVE_PRIORITY 100
 
 /* Hash table parameters */
 #define REDIS_HT_MINFILL        10      /* Minimal hash table fill 10% */
@@ -479,6 +480,7 @@ struct redisServer {
     time_t repl_transfer_lastio; /* unix time of the latest read, for timeout */
     int repl_serve_stale_data; /* Serve stale data when link is down? */
     time_t repl_down_since; /* unix time at which link with master went down */
+    int slave_priority;             /* Reported in INFO and used by Sentinel. */
     /* Limits */
     unsigned int maxclients;
     unsigned long long maxmemory;