Don't expire keys when loading an RDB after a SYNC

The cron is responsible for expiring keys. When keys are expired at
load time, it is possible that the snapshot of a master node gets
modified. This can in turn lead to inconsistencies in the data set.

A more concrete example of this behavior follows. A user reported a
slave that would show an monotonically increase input buffer length,
shortly after completing a SYNC. Also, `INFO` output showed a single
blocked client, which could only be the master link. Investigation
showed that indeed the `BRPOP` command was fed by the master. This
command can only end up in the stream of write operations when it did
NOT block, and effectively executed `RPOP`. However, when the key
involved in the `BRPOP` is expired BEFORE the command is executed, the
client executing it will block. The client in this case, is the master
link.

src/rdb.c

@@ -993,8 +993,12 @@ int rdbLoad(char *filename) {
         if ((key = rdbLoadStringObject(fp)) == NULL) goto eoferr;
         /* Read value */
         if ((val = rdbLoadObject(type,fp)) == NULL) goto eoferr;
-        /* Check if the key already expired */
-        if (expiretime != -1 && expiretime < now) {
+        /* Check if the key already expired. This function is used when loading
+         * an RDB file from disk, either at startup, or when an RDB was
+         * received from the master. In the latter case, the master is
+         * responsible for key expiry. If we would expire keys here, the
+         * snapshot taken by the master may not be reflected on the slave. */
+        if (server.masterhost == NULL && expiretime != -1 && expiretime < now) {
             decrRefCount(key);
             decrRefCount(val);
             continue;