Throttle BGSAVE attempt on saving error.

When a BGSAVE fails, Redis used to flood itself trying to BGSAVE at
every next cron call, that is either 10 or 100 times per second
depending on configuration and server version.

This commit does not allow a new automatic BGSAVE attempt to be
performed before a few seconds delay (currently 5).

This avoids both the auto-flood problem and filling the disk with
logs at a serious rate.

The five seconds limit, considering a log entry of 200 bytes, will use
less than 4 MB of disk space per day that is reasonable, the sysadmin
should notice before of catastrofic events especially since by default
Redis will stop serving write queries after the first failed BGSAVE.

This fixes issue #849

src/redis.h

@@ -97,6 +97,7 @@
 #define REDIS_DEFAULT_REPL_BACKLOG_SIZE (1024*1024)    /* 1mb */
 #define REDIS_DEFAULT_REPL_BACKLOG_TIME_LIMIT (60*60)  /* 1 hour */
 #define REDIS_REPL_BACKLOG_MIN_SIZE (1024*16)          /* 16k */
+#define REDIS_BGSAVE_RETRY_DELAY 5 /* Wait a few secs before trying again. */
 
 /* Protocol and I/O related defines */
 #define REDIS_MAX_QUERYBUF_LEN  (1024*1024*1024) /* 1GB max query buffer. */
@@ -616,6 +617,7 @@ struct redisServer {
     int rdb_compression;            /* Use compression in RDB? */
     int rdb_checksum;               /* Use RDB checksum? */
     time_t lastsave;                /* Unix time of last successful save */
+    time_t lastbgsave_try;          /* Unix time of last attempted bgsave */
     time_t rdb_save_time_last;      /* Time used by last RDB save run. */
     time_t rdb_save_time_start;     /* Current RDB save start time. */
     int lastbgsave_status;          /* REDIS_OK or REDIS_ERR */